I have this same problem. The device has worked flawlessly for a couple of months, but now shows offline in RMS whatever I do. It has credits etc, and it is providing internet service to attached devices, but no RMS connection. I have tried using SMS UCI API messages to turn monitoring off and on, but still shows offline in RMS. SMS monitoring_status commands returns Monitoring-enabled;Connection-closed.
Seems like this is a problem for many people. It is a shame, because otherwise the system is superb, and I was planning to deploy n several locations, but this problem may make them useless for me.