Modem not trying other PLMN's if nw_mode is not included in the dataplan?

[JyriLehtinen]

We're currently using a virtual operator as carrier, so our device is roaming in Finland. It seems that Elisa (24405) is the preferred network, but the data plan is restricted to LTE-M on Elisa network. The SIM does have NB-IoT enabled on DNA network (24412), and I can connect using NB-IoT and calling AT+COPS=1,2,"24412" manually.

It seems that the modem doesn't automatically try other networks after not being able to register to the primary. Is this an issue with the modem, network or the carrier profile?

Attached is the modem trace (Ignore the flight mode after 15 minutes, it is intentional). Modem fw 1.3.6, SDK version 2.6.1.

2024-09-17_ttyACM2_trace.bin 

[Øyvind]

Hello, 

Thank you for providing modem traces and information on modem fw version. I will have a look at the trace. What SIM card are you using?

Kind regards,
Øyvind

[JyriLehtinen]

The operator is 1oT

Plan is 1oT Global eUICC, "optimal coverage"

[Øyvind]

From the modem trace it is clear that your device is rejected from the NB-IoT network

CEREG: 2,"BFE1","00120A67",9,0,15

Cause #15 – No suitable cells in tracking area
This EMM cause is sent to the UE if it requests service, or if the network initiates a detach request, in a tracking
area where the UE, by subscription, is not allowed to operate, but when it should find another allowed tracking
area or location area in the same PLMN or an equivalent PLMN.

Kind regards,
Øyvind

[JyriLehtinen]

Right, makes sense. However, the modem does not connect to other allowed PLMN's in this case, but remains disconnected until LTE-M is enabled or PLMN is selected manually with AT+COPS.

[Øyvind]

Reject cause is #15 “No suitable cells in tracking area” is sent by network when the subscription does not allow operation in the tracking area of this network. In automatic network selection mode (AT+COPS=0) modem can try to Attach also to other networks in the area. In manual network selection mode modem just tries to find another, allowed tracking area within the same, manually selected network.

[JyriLehtinen]

But in my case, it does not automatically try to attach to other networks, even though it should

[Øyvind]

Have you issued AT+COPS=0

Note: The set command selects a mobile network automatically or manually. The command configuration is stored to NVM approximately every 48 hours and when the modem is set to minimum functionality mode with the +CFUN=0 command.

[JyriLehtinen]

The application does not explicitly call AT+COPS=0, it uses lte_lc_connect_async().

I can try if adding the AT+COPS=0 works.

[JyriLehtinen]

Calling AT+COPS=0 seemed to do the trick, device now connected to DNA network on NB-IoT. Without the AT command it was offline for days until I terminated the test.

Is there a reason why lte_lc_connect_async() doesn't behave the same or is something misbehaving?

[Øyvind]

Thanks for clarifying! Happy to hear that it is working as intended.

I have forwarded the question and hope to have an answer by tomorrow.

Kind regards.
Øyvind