There is a long thread in the Google support forums which links to several other long threads regarding Google Pixel phones with Android Oreo 8.0 or 8.1 repeatedly disconnecting from WiFi access points that work for other users. There are multiple causes, some of which were fixed in the January security patch, so here’s the TL;DR. (As you can see, factory resetting should be a last resort rather than a first resort in this case.)
- Delete Verizon “Security & Privacy” app, or at least turn off WiFi security, on branded phones.
- Disable all Chromecast devices on the network to prevent a flood condition when the phone first comes online. This problem was fixed in the January security patch.
- Disable 5GHz on a dual-band wireless access point.
- Set a static IP for the phone in the router DHCP configuration.
- Use WPA to secure your wireless access point instead of the obsolete WEP standard.
- Turn on Bluetooth.
- Disable IPv6 in the router configuration.
- Reboot in ‘safe mode’, then disable and re-enable WiFi and see if it stays connected. If it does, reboot normally and then:
- Force-stop the Google Home app, disable and re-enable WiFi.
- Force-stop the Waze app, disable and re-enable WiFi. (This was the fix for my phone on Oreo 8.1, as improbable as it may be.)