I also have the same issue of the MG-90 Restarting my Cellular interface.
This ONLY occurs when the MTU Size is modified on the WAN Cellular Link from 1500 to 1400.
I need the ability to have a MTU size of 1400 to connect VPN over Cellular to Microsoft Azure VPN Gateway
Remove all monitors and still have same issues. Changing the MTU back to 1500 , the link stays up, but cannot connect to VPN
Any ideas ?