Were you able to connect previously?
Did you perform firmware update with Keep settings option set to on recently?
Could you perform device's factory reset and see if that helps? (In the WebUI, navigate to System -> Backup -> Restore default settings)
Have you tried to use custom APN value? (Network -> Interfaces -> <Mobile_Interface_Settings>)
Otherwise, if the above do 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.