I been having a weird issue the last couple of weeks. I have a deco m5 setup connected to a SB8200 with xfinity. 

Deco connection has been solid for over 3 years now, except for the past two weeks. The modem will be online, but the deco would not recognize that it is active. 

Rebooted the deco network and everything worked fine. A week later on the same day (Monday), deco network was showing offline. Modem was active. I rebooted just the main deco this time (connected to the modem) and everything went back to normal. 

I logged in to the modem, and uptime indicated that the device was active for 7 days (since last reboot).

Can provide the log if necessary.