Hi,
Got a similar problem in 6.08.6, my router appear in "Not activated" status on RMS (grey), and i got a Device is not registered in RMS error on the router side.
The router was set factory and got a valid WAN connection. But no way to make it appear online.
I was stuck on this for 1h until finding this thread. It seems that the previous solution is working for that case too. (Disable monitoring, THEN set factory)
Hope it can help someone else !
Best regards.