This problem started after I upgraded a RUT950 to firmware version RUT9_R_00.07.04.1. The RMS portal shows the device as offline, but when I log in to the device UI (I have it connected to a ZeroTier VPN) it shows it's connected to RMS.
I can resolve the problem by unregistering and re-registering the device to RMS or disabling then enabling RMS via the device UI, after which RMS shows it as online for about a day, then goes back to saying the device is offline. The device UI still says it's connected to RMS.