Is 5G available in your area?
What is the operator of your mobile network?
Does your mobile plan include 5G?
Could you check up with your provider, if 5G does not require any custom APN?
Otherwise, I would like you to configure your router's mobile mode to 4G+5G, leave the router running for some time, for it to disconnect/reboot, then generate a troubleshoot file and attach it by editing your question. Make sure the issue is present before generating the file, 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.