Hello,
Thank you for reaching out!
I will ask you to replicate the issue, log into the WebUI of the router, navigate to System → Administration → Troubleshoot, and generate a troubleshoot file. This file can be attached to the original post and will only be visible to Teltonika moderators.
This file will contain logs that will help determine the reason why the device is not connecting to the mobile network.
Best regards,
DaumantasG