Hello,
Since you were able to reach the login page, it does not seem to be a firewall issue, but just in case, could you ask your provider if no additional ports are needed to be open on router's firewall, in order to proceed with the connection?
Are you able to login to the device locally?
For troubleshooting, could you login to the device via SSH, execute command below and then try to login from mobile WAN? Check, if login attempts do not generate any log entries?
For additional details, could you download TCPdump from router's WebUI Services -> Package manager -> Packages page and enable TCP dump monitoring in System -> Administration -> Troubleshoot page, specifying wwan0 (mob1s1a1_4) interface? Try a few login attempts from your mobile network to generate some traffic. Download capture file from the same Troubleshoot page and attach it by editing your question.
Lastly, 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,