Hello,
Have you tried to set custom APN?
It can be set in Network -> Interfaces page in the WebUI by editing mobile interface settings and disabling Auto APN slider button.
Otherwise, it would be helpful, if you could provide a troubleshoot file. Please, replicate the issue, then access router's WebUI, go to System -> Administration -> Troubleshoot section, download troubleshoot file from there and attach it by editing your question.
Best regards,