A new troubleshoot file would be needed from your device to check for possible causes, however, the first thing I would suggest to try is to configure custom APN for mobile interface.
To do it, if by the latest firmware you mean 7.02.7, simply access router's WebUI, navigate to Network -> Interfaces, edit mobile interface settings, disable Auto APN slider button, set APN to -- Custom -- and enter custom value according to your service provider. See if that helps.
Best regards,