RBR40 main hub keeps going offline even tough it is connected by ethernet to xfinity modem

I am having the exact same issue. I have a timed outlet that actually reboots my router nightly (around 3:30am) and reboots it. That had resolved some annoying issues but over the last few days every single morning, I have internet for about 15 minutes and then the main router connected to my AT&T router w/ethernet goes offline. Firmware up to date according to the search I just did. Extremely frustrating and annoying as I'm having to reboot 2-3 times a day. That can't be the solution.