A short digest of the parenting topic (https://devzone.nordicsemi.com/f/nordic-q-a/76906/nrf9160-siba-unable-to-connect-to-nb-iot-network-and-resets-every-10-seconds:)
Custom board with nRF9160 SIBA chip (NB-IoT network only, no GPS, no LTE-M)
Modem firmware 1.3.0
Russian region, MTS cellular provider
The chip is unable to establish data connection.
AT%XMONITOR outputs valid cell ids (only digits, no text labels) with status 2 - "Not registered, but UE is currently trying to attach or searching an operator to register to"
AT%CESQ shows signal levels of ..,25,40