Hello,
I'm assuming you're using "Auto APN" instead of custom APN? If so, could you please try setting APN given by your provider?
You can check if APN is set to auto or custom in the mobile settings here: Network > Interfaces > MOB1S1A1 or MOB2S2A1. Make sure that correct APN is entered. If you're not sure or don't know this information, please search online for that operator’s SIM card setting, or ask your mobile operator for this information.
Additionally, in the Network>Mobile settings, please make sure the correct PIN code has been entered for the respective card.
Also, in case you're using older firmware, please download and install the latest 7.1 firmware without keeping router settings. You can download it from our wiki here: https://wiki.teltonika-networks.com/view/RUTX12_Firmware_Downloads
If you're still having problems let me know and I'll help you to troubleshoot this issue further.
Best regards,
Tomas.