Hello,
Could you try to update to the latest firmware RUT2_R_00.07.02.2, which you can download from here?
If you are using an auto APN option, I would like you to try to set APN manually. You can do it by logging to the web interface, navigating to Network -> Interfaces and editing your mobile interface settings by pressing a pencil icon. In a new configuration window set Auto APN slider button to off, select -- Custom -- APN and enter your APN below.
If that does not help, I would like you to attach a troubleshoot file to your question. Please, replicate the issue, then access router's WebUI, go to System -> Administration > Troubleshoot section and download troubleshoot file from there.
Best regards,
Žygimantas