nRF9151 puts wrong network in FPLMN

[Matiss]

Hello,

We have an issue. Our device is a custom board based on the nRF9151, and we use it in Latvia. Two network providers are available for our Onomondo SIM card: one supports LTE-M/NB-IoT and the other doesn't. When both networks are enabled in the Onomondo network list, the device uses the network that supports LTE-M/NB-IoT and works correctly.

However, when we remove the LTE-M/NB-IoT network from the Onomondo network list and leave only the other one enabled, we start getting cause 15 and cause 11 after a reboot. Then the strange part begins: when we check which network has been added to the SIM card's FPLMN (forbidden networks) list, it is the LTE-M/NB-IoT network (which is not even enabled in Onomondo) instead of the other one (which is enabled in Onomondo). After that, even if we re-enable the LTE-M/NB-IoT network, the device still gets cause 15 and does not work.

Also, when I run a network scan with the modem's AT+COPS=? command, it shows the LTE-M/NB-IoT network and a third network that Onomondo doesn't support at all. The other network in our Onomondo list is not visible. Another thing, the third network never gets put on SIM card's FPLMN (forbidden networks) list.

What could be the reason for the wrong network being blacklisted, and is there a way to prevent this?

[Syed Maysum Abbas Zaidi]

Hi,

Thanks for reaching out.

Actually removing the LTE-M/NB-IoT network from the Onomondo network list does not have prevented an attach attempt to the network so most likely an attempt was made but it resulted in a PLMN not allowed (Cause #11) as it was not enabled through Onomondo.

Then under the 3GPP standard, a reject with Cause #11 puts that network on the SIM's forbidden network list (FPLMN) so now even when you enabled through the Onomodo, it was rejected. Manual PLMN selection with +COPS should be able to remove the it from the forbidden list.

However we can check it properly if you can share the modem trace and your AT logs.

Best Regards,
Syed Maysum