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

[gav_ncl]

I would like to evaluate the use of NB-IoT in the UK which Wireless Logic SIM included with nrf9151DK provides. Using the following sequence of AT commands I can send data over LTE-M to thingspeak but if I try the same sequence with NB-IoT as the mode and preference (i.e. AT%XSYSTEMMODE=0,1,0,2) i get ERROR response when i try to connect using AT#XMQTTCON instead of OK when using LTE-M. Can you advise where I may be going wrong please? I am using the Serial LTE Modem application in nrfConnect. 

• AT#XRESET
• AT%XSYSTEMMODE=1,1,0,1
• AT+CFUN=1
• AT+CEREG=5
• AT+CSCON=1
• AT+CGDCONT=1,"IP","APN HERE"
• AT+CESQ=0
• AT#XMQTTCFG="CLIENT ID HERE",300,1
• AT#XMQTTCON=1,"USER NAME HERE","PASSWORD HERE","mqtt3.thingspeak.com",1883
• AT#XMQTTPUB="TOPIC HERE","DATA HERE"

[JONATHAN LL]

Hi,

What is the error? 

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

Regards,
Jonathan

[gav_ncl]

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

[JONATHAN LL]

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

[gav_ncl]

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

trace-2025-10-29T10-43-01.556Z.mtrace

[JONATHAN LL]

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