Hello,
Does the mobile connectivity remain unstable, when a single card is used?
Have you tried setting up a custom APN value in Network -> Interfaces -> Mobile interface settings section?
Aside from that, 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,