Hello,
Bootloader is unlikely to be an issue here.
Have you checked, if the issue is specific to SIM slot 1 or a specific operator, or SIM card?
Does the loss of connection recover by itself or do you have to reset the device?
Could you please install 7.03.4 version by WebUI? Make sure you update with Keep settings set to off. This will delete your configuration, but you will have a clean install.
Then I would like you to generate a troubleshoot file and attach it by editing your question. Please, replicate the issue, that is the router must have recently experienced a loss of mobile connectivity, then access router's WebUI, go to System -> Administration -> Troubleshoot section and download troubleshoot file from there.
The logs in the file might provide more insight into the issue.
Best regards,