nRF9160 LTE-M network registration issue

[Fryderyk]

Hello,

we are experiencing an issue with devices based on nRF9160 using modem firmware lib 1.3.6 and nRF Connect SDK 2.5.3.

The problem manifests as a sudden inability to register to the LTE-M network. After successful initialization, the device waits for the
LTE_LC_EVT_NW_REG_STATUS event for approximately 15 minutes, but the event never occurs. Restarting the device does not resolve the issue.

The situation is only resolved after inserting a different SIM card, after which the device registers to the network correctly. The SIM card that did not work on our device was tested:

• on multiple units of our devices,

• in different locations,

and the issue was reproducible in all cases.

Importantly, the same SIM card was able to successfully register to the network only after being used in an nRF9160 DK with the Cellular Monitor v2.6.0 (modem firmware lib 1.3.5 )application. From that point on, the SIM card also started working correctly in our devices and the issue no longer occurs.

At the moment, due to the inability to reproduce the issue, it is difficult for us to debug the root cause, which is still unknown. We would like to ask:

• how we should further approach debugging this problem,

• what could be the potential causes of this behavior (e.g. SIM state, network behavior, modem state).

We are attaching a trace file captured using Cellular Monitor on the nRF9160 DK, recorded at the moment when the SIM card transitioned from a state in which it was unable to register to the network to a successful LTE-M network registration.trace-2026-01-16T11-26-16.868Z.mtrace

Thank you in advance for your support and guidance.

Best regards,

[Achim Kraus]

Fryderyk said:

Restarting the device does not resolve the issue.

Of course just a "guessing game" and work-around: die you try a "AT%XFACTORYRESET=0"?

AFAIK, the modem keeps some state related to the network and SIM (otherwise it would not restart that fast). Exchanging the SIM may reset that state, and so the device may be able to connect to the radio network again. A "AT%XFACTORYRESET=0" may also have a chance to reset such state. Maybe at least worth to try.

[Fryderyk]

Hi,

thank you — these are exactly the kind of suggestions we are looking for.

We have not tried AT%XFACTORYRESET=0 yet, but this is a very good hint and we will definitely test it as a potential recovery mechanism in our firmware.

Would it be possible to obtain (or is there available documentation for) the AT command sequence used by Cellular Monitor during initialization and network registration? We would like to analyze which actions are performed there that could be responsible for bringing the SIM card back to a working state.

[Achim Kraus]

I'm an other user. But maybe Øyvind from Nordic can help here.

[Øyvind]

Hello, 

Fryderyk said:

We have not tried AT%XFACTORYRESET=0 yet, but this is a very good hint and we will definitely test it as a potential recovery mechanism in our firmware.

Have you been able to try this yet?

We are attaching a trace file captured using Cellular Monitor on the nRF9160 DK

As you are using modem FW 1.3.5, are you able to test with modem FW 1.3.7 to ensure any possible fixes are included? 

Fryderyk said:

We have been using SIM cards from the same distributor in Poland (Orange)

Have you been in contact with the distributor and ensured that there is no issue with subscription or similar? As the device has been running stable for three months it does sound like an issue with the subscription. But we will need modem trace to verify or some logs, I'm afraid.

Will forward modem trace to our experts to review. 

Kind regards,
Øyvind

[Øyvind]

Fryderyk said:

Would it be possible to obtain (or is there available documentation for) the AT command sequence used by Cellular Monitor during initialization and network registration?

Sorry, forgot to answer this question. There is documentation available for the Cellular Monitor app. This does not state directly AT commands are used, but opening a serial terminal and clicking refresh dashboard will show the commands in clear text in serial terminal for you to copy. 

Kind regards,
Øyvind