Recurring and seemingly random loss of internet access, despite multiple troubleshooting calls.

I have been experiencing repeat episodes of degraded network functionality outside of my local environment. 

For background, my system is set up in the following way;

AT&T fiber service --> AT&T supplied modem --> Google OnHub router, which feeds a Google WiFi mesh network with a total of 3 hub points.

The fiber service was initiated in May/June 2017 and the above home mesh network was set up the following day. This system worked flawlessly for approximately 3-4 months. At that point, I began to notice degraded functionality for no apparent reason, as the device settings had not been changed and no alterations were made to the hardware configuration (beyond adding client devices).

Making this more confusing, the issue always comes about at some point during the night. Each occurrence is random and my network may go 1 day to a couple of weeks before the problem repeats. With each episode, I spend the evening enjoying normal functionality (able to stream UHD video, browse the web, check email; all done simultaneously with no problems detected); go to bed; wake up the following morning to find that there is no communication across the router and to the outside web.

To date, I have reset the modem and Google router to factory settings. Also, a home service call was performed Dec 29, 2017, at which time my modem was changed to the Arris BGW210-700 model and the technician informed me that any changes to SSID/password/etc would result in loss of function from the modem (which seems asinine, but that's a different story). Following the instructions provided here: BGW210 passthrough mode, I setup passthrough mode and enjoyed flawless home network service through my Google mesh system until Jan 9, 2018, at which point I awoke to find the data transmission outside of my local wifi environment was again disrupted.

At this point, I am extremely frustrated and confused by the seemingly random nature of these events. Multiple hours worth of tech calls to troubleshoot the issue have been useless, as the service reps 1)  consistently tell that enabling passthrough mode will render my network non-functional, and 2) sound completely clueless with regard to the underlying issue.

Any assistance or thoughts about how to work toward resolving this problem will be greatly appreciated.