I recently had an issue where my modem router needed to be replaced, after which my Google Mesh Wifi failed to communicate with the new router. I have performed a factory reset of the mesh router, and after several attempts, it is now connecting and I can then connect devices to it (and subsequently to the internet). When I attempt to re-add the other mesh wifi points (which I have also factory reset) via the Google Home app, they connect, but when the app attempts to add them to the mesh network, they both fail saying "something went wrong. Please ensure that you are connected to the internet and try again." The device I'm running the app from is connected and able to access the internet, so I've got no idea on how to fix this.
Well that was quite possibly the most painful process I've ever been through. Replacing my modem router resulted in me needing to completely reset my Google nest wifi router twice, disconnect and reconnect my Google cloud services, restart all Google NEst wifi devices as well as my modem router (with quite literally 8 factory resets of Google mesh wifi points throughout the process).
There's no way anyone who is not technically savvy would be able to do this. Come on Google, you can do way better than this.