Please go to System --> Administration --> Troubleshoot and download the troubleshoot file.
Once you get the file, open system.log and search the section GSM information.
If your router is recognizing your SIM card should appear in this section..
(For this example, I`m not using SIM card, and the issue should appear in this way.)
If the router it`s not recognizing the sim card, check the physical slot.
Else, you can check the mobile configuration in Network --> Mobile