MQTT connection fails over NB-IoT using nRF9151 DK and Serial Modem

RE: MQTT connection fails over NB-IoT using nRF9151 DK and Serial Modem

Håkon Alseth — Wed, 11 Feb 2026 15:31:47 GMT

Hi Federica,

[quote user="feda"]Thank you for your support.
I was able to solve the problem by reviewing other tickets on this portal: I found that the device needed to be configured to use PCO instead of EPCO (which is the default option in the Serial Modem firmware). After setting the command AT%XEPCO=0, the MQTT connection was successfully established.[/quote]

Glad to hear that you found the root cause of the issue.

[quote user="feda"]For this reason, I would appreciate receiving documentation or reference material regarding CoAP management via AT commands. Unlike MQTT, I didn't find a dedicated section for CoAP in the Serial Modem AT command documentation.

Any guidance or documentation you can share would be greatly appreciated.[/quote]

Serial Modem does not expose a CoAP AT command set, unfortunately. Here is the overall socket commands that are covered:

https://docs.nordicsemi.com/bundle/addon-serial_modem-latest/page/index.html

Specific protocols, like lwm2m carrier library, are possible to enable, but these are higher level application specific functions.

However, you can setup a generic udp (datagram) socket.

An alternative is to use the coap client:

https://docs.nordicsemi.com/bundle/ncs-3.2.2/page/nrf/samples/net/coap_client/README.html

Kind regards,

Håkon

RE: MQTT connection fails over NB-IoT using nRF9151 DK and Serial Modem

feda — Wed, 11 Feb 2026 09:23:26 GMT

Hi Håkon,

Thank you for your support.
I was able to solve the problem by reviewing other tickets on this portal: I found that the device needed to be configured to use PCO instead of EPCO (which is the default option in the Serial Modem firmware). After setting the command AT%XEPCO=0, the MQTT connection was successfully established.

At the moment, I have not experienced any timeout-related issues. However, since the system will need to be replicated on a larger scale, it would be better to prevent future issues.

For this reason, I would appreciate receiving documentation or reference material regarding CoAP management via AT commands. Unlike MQTT, I didn't find a dedicated section for CoAP in the Serial Modem AT command documentation.

Any guidance or documentation you can share would be greatly appreciated.

Best regards,
Federica

RE: MQTT connection fails over NB-IoT using nRF9151 DK and Serial Modem

Håkon Alseth — Mon, 09 Feb 2026 12:38:42 GMT

Hi Federica,

Could you share a log of what occurs (and possibly a modem trace)?

If the RSRP (signal strength) is low, it can be problematic as you will get a heavily reduced overall throughput if retransmissions occur on-air (CE level increases).

[quote user=""]Are there known limitations or specific requirements when using MQTT over NB-IoT with the Serial Modem on nRF9151?[/quote]

TCP based communication tend to suffer in terms of timeout, due to the lower overall throughput on NB-IoT as compared to LTE M1.

Other than that, there is no technical reason why it should not work; but you will see a difference in terms of robustness, especially when coverage is bad.

Kind regards,

Håkon