- ubus call rms get_status | grep status
should return a value of 0 in case the router is connected to RMS and a 1, if not, along with error codes and explanatory texts.
If it does not display any errors, from the router's perspective, it is connected to RMS. I also doubt, if any logs are given in such cases.
Since I have not experienced a similar situation, you can try stopping rms process in the router by /etc/init.d/rms_mqtt stop and running it in debug mode with rms_mqtt -d &, to see, if anything shows up.
I would also like to note, that there were some issues with RMS recently, a more major one had occurred during the weekend, which resulted in some devices not being able to come up online in the system. It has now been resolved, and devices should be reconnecting back to the system correctly.
In your case, are there any devices at the moment, that are still not connecting back to RMS? Could you send their serial numbers in a private message?