NB-IoT post data using AT Commands and MQTT to ThingSpeak

RE: NB-IoT post data using AT Commands and MQTT to ThingSpeak

gav_ncl — Mon, 24 Nov 2025 14:25:09 GMT

Hi Hakon,

Thanks for your response. Setting AT%XPCO=1 now enable me to send data via MQTT using NB-IoT.

I will make the above as accepted answer to my query.

One other question, I notice occasional long periods when I am querying AT+CEREG? and I get +CEREG: 5,0. Why would the modem be "Not registered. User Equipment (UE) is not currently searching for an operator to register to."? How can I command the modem to search when it is already active with CFUN=1 and the other settings I have used above? In my use case I want to power up the unit, find a connection, send the data and then power down so periods where it isnt searching uses power and wastes time.

Thanks,

Gavin

RE: NB-IoT post data using AT Commands and MQTT to ThingSpeak

Hakon — Fri, 21 Nov 2025 09:05:43 GMT

You can request the following notification Set command (AT%XPCO=1) and if received Notification is empty → NB-IoT ePCO is not supported in the SIM provider Packet Gateway element.

Set command AT%XEPCO=0 should fix this situation with Serial Modem application.

RE: NB-IoT post data using AT Commands and MQTT to ThingSpeak

gav_ncl — Wed, 19 Nov 2025 11:23:30 GMT

Hi Hakon,

It is an instant error. I have tried multiple brokers including the one in your example. LTE-M works on every broker but I have not been able to connect to a broker using NB-IoT.

Do you have any idea what I may be doing wrong? I can see that the unit is connected via NB-IoT no problem but for whatever reason it will not connect to an MQTT broker.

Thanks

Gavin

RE: NB-IoT post data using AT Commands and MQTT to ThingSpeak

Hakon — Wed, 19 Nov 2025 09:35:18 GMT

[quote user="gav_ncl"]I have tried your AT commands and similar to your second attempt it wont connect and produces ERROR. It would be good if it said something more than just ERROR.[/quote]

Does it time out, or is it an instant error? If it times out, then you should try a different broker.

[quote user="gav_ncl"]Does the latest firmware mfw_nrf91x1_2.0.2 support MQTT for NB-IoT using AT Commands?[/quote]

It should be supported as far as I know.

RE: NB-IoT post data using AT Commands and MQTT to ThingSpeak

gav_ncl — Mon, 17 Nov 2025 21:19:28 GMT

Hi Hakon,

I have tried your AT commands and similar to your second attempt it wont connect and produces ERROR. It would be good if it said something more than just ERROR.

Does the latest firmware mfw_nrf91x1_2.0.2 support MQTT for NB-IoT using AT Commands?

After trying for some time I have still been unable to send a single message via NB-IoT using MQTT yet LTE-M works perfect.

Do you have any further advice to get this working?

Thanks again,

Gavin

RE: NB-IoT post data using AT Commands and MQTT to ThingSpeak

Hakon — Thu, 13 Nov 2025 10:03:17 GMT

It is pretty much exactly what you did, setting AT%XSYSTEMMODE=0,1,0,2. So setting something like

AT%XSYSTEMMODE=0,1,0,2

at+cfun=1

at#xmqttcfg="something123",300,1

at#xmqttcon=1,"","","test.mosquitto.org",1883

OK

#XMQTTEVT: 0,0

+CEREG: 5,"A879","014A0303",9,,,"11100000","11100000"

at%xsystemmode?

%XSYSTEMMODE: 0,1,0,2

The broker seems to be down at the moment, but it was up and working when I was testing it earlier.

RE: NB-IoT post data using AT Commands and MQTT to ThingSpeak

gav_ncl — Mon, 10 Nov 2025 15:53:51 GMT

Can you include all of your setup AT commands so that I can try to respoduce your result? This cant be everything as you need to turn the modem on.

Thanks

Gavin

RE: NB-IoT post data using AT Commands and MQTT to ThingSpeak

Hakon — Tue, 04 Nov 2025 10:04:42 GMT

I have not been able to use the same setup as you, but just testing with basic public brokers works just fine;

at#xmqttcon=1,"","","test.mosquitto.org",1883

OK

#XMQTTEVT: 0,0

+CEREG: 5,"A879","014A0303",9,,,"11100000","11100000"

at%xsystemmode?

%XSYSTEMMODE: 0,1,0,2

I would assume this basic test should also work for you. If it does, then maybe I can try to replicate your setup more closely. If necessary, we can make the ticket private, so you can share more details.

RE: NB-IoT post data using AT Commands and MQTT to ThingSpeak

gav_ncl — Mon, 03 Nov 2025 11:32:07 GMT

Ok no problem. Seems strange that LTE-M works no problem with the AT commands above and changing only AT%XSYSTEMMODE to NB-IoT is enough to stop it working. Using the above set of AT commands with AT%XSYSTEMMODE=0,1,0,2 can you send data using MQTT over NB-IoT with the SLM sample?

Thanks

Gavin

RE: NB-IoT post data using AT Commands and MQTT to ThingSpeak

Hakon — Mon, 03 Nov 2025 11:23:48 GMT

[quote user="gav_ncl"]What information did the trace I attached for Jonathan show?[/quote]

From the information you provided I would think that the error is related somehow to the AT handler, and that the error would likely not show up in the modem logs at all. If you give me some time I will try to reproduce the error.

RE: NB-IoT post data using AT Commands and MQTT to ThingSpeak

gav_ncl — Mon, 03 Nov 2025 10:13:10 GMT

Hi Hakon,

No modifications to the SLM code. Only interraction is using AT commands. I have since confirmed with the SIM provider that TCP using NB-IoT is supported. What information did the trace I attached for Jonathan show?

Thanks

Gavin

RE: NB-IoT post data using AT Commands and MQTT to ThingSpeak

Hakon — Mon, 03 Nov 2025 10:07:33 GMT

Hello,

are you running the SLM sample as it is, or have you made any modifications in the code?

RE: NB-IoT post data using AT Commands and MQTT to ThingSpeak

gav_ncl — Thu, 30 Oct 2025 11:57:09 GMT

Hi Jonathan,

I have ran the MQTT sample with the modifications specified. Attached is the modem trace.

Thanks

Gavin

devzone.nordicsemi.com/.../trace_2D00_2025_2D00_10_2D00_30T10_2D00_28_2D00_35.347Z.mtrace

RE: NB-IoT post data using AT Commands and MQTT to ThingSpeak

JONATHAN LL — Wed, 29 Oct 2025 22:33:24 GMT

Thanks,

Stil plausible that this might be a issue with access on NB-IoT. The trace does not seem to show any IP traffic, so it might be that TCP is not fully supported where you are.

Can you test the MQTT sample with tracing this should provide us with more info.

https://docs.nordicsemi.com/bundle/ncs-latest/page/nrf/samples/net/mqtt/doc/description.html
https://github.com/nrfconnect/sdk-nrf/tree/main/samples/net/mqtt

When building with the VScode extension you add the extra CMAKE argument: Dmqtt_SNIPPET=nrf91-modem-trace-uart

You might also ned to set CONFIG_LTE_NETWORK_MODE_NBIOT in the lte config to make sure NB-IoT is used.

This should provide some more info and the trace from this will also clear up what exactly is happening.

Regards,
Jonathan

RE: NB-IoT post data using AT Commands and MQTT to ThingSpeak

gav_ncl — Wed, 29 Oct 2025 10:55:02 GMT

Hi Jonathan,

Thanks for your response.

I only get the response ERROR when trying to use NB-IoT but I do get a response when connecting via LTE-M.

I have tried to capture a trace. I am new to this so I hope it is correct. I basically appied the AT commands listed above along with AT%XMODEMTRACE=1,2. I tried a few times to connect with AT#XMQTTCON but again received the response ERROR. Judging by the cellular monitor it appears to be connect via NB-IoT but just cant connect to MQTT broker but this does work with LTE-M.

Trace file is attached.

Thanks for your help.

Gavin

devzone.nordicsemi.com/.../trace_2D00_2025_2D00_10_2D00_29T10_2D00_43_2D00_01.556Z.mtrace

RE: NB-IoT post data using AT Commands and MQTT to ThingSpeak

JONATHAN LL — Tue, 28 Oct 2025 22:54:16 GMT

Hi Gavin,
I believe you should get a response according to the doc for the AT command. https://docs.nordicsemi.com/bundle/ncs-latest/page/nrf/applications/serial_lte_modem/doc/MQTT_AT_commands.html#d410e1109

#XMQTTEVT=<evt_type>,<result>

giving you a negative number for the ERROR reason.

Does it just print ERROR and nothing else?

I can do modem trace here and we can have a look to see if there is any other issue, it should tell us what happens.
https://academy.nordicsemi.com/courses/cellular-iot-fundamentals/lessons/lesson-7-cellular-fundamentals/topic/lesson-7-modem-trace

Regards,
Jonathan

RE: NB-IoT post data using AT Commands and MQTT to ThingSpeak

gav_ncl — Tue, 28 Oct 2025 15:05:14 GMT

Hi Jonathan,

Unfortunately I dont get any further information in the response from the AT#XMQTTCON command other than "ERROR" which is frustrating. I dont know if there is a AT command which enables further diagnostic information but at the moment I just get ERROR. Using AT%XSYSTEMMODE=1,1,0,1 it works perfectly and the response is "OK" and can continue to publish data. I cant see it being any sort of syntax error as the only thing I change in the process is to select NB-IoT mode only. I checked with my SIM provider and NB-IoT is definately supported and the APN is correct.

Thanks,

Gavin

RE: NB-IoT post data using AT Commands and MQTT to ThingSpeak

JONATHAN LL — Tue, 28 Oct 2025 14:20:39 GMT

Hi,

What is the error?

Can you share the error message so that we can see the actual error response?

Regards,
Jonathan