Not a lot to go on there, but the challenge sounds similar to what I worked through for my Netgear CM1200 modem. The Nest WiFi router handles the IP routing load, but handoff to the modem will vary depending upon the modem. My CM1200 has 4 LAN ports, but only the first port can actually function for connection to the router. Check your documentation for any similar issues for your modem. Though mine was undocumented, that and the required mystic order of booting and connecting cables, was discovered on yet another forum. The ONLY device that can connect to the modem is the Nest router. Any downstream wired LAN switches, bridges, or concentrators must connect to the second port on the Nest router (assuming it still exists on new models.)
This works for my modem and it sometimes requires several tries as you'll likely be totally blind to both what's actually happening upstream of your modem and anywhere within the Nest router. Some of the mysterious parts are likely what's happening (also out of sight) upstream with my Xfinity provider handling its end of the CM1200 interface.
The most common problem I see after restart is the PC or phone WiFi connects to the Nest router, but fails to see beyond. Go back to either of Step 1 or Step 3, above. Rinse and repeat as needed until successful.
Most frustrating to me is the complete lack of helpful status from the Google nodes at-large. The WiFi points are arrogant products that consumers are told will just work. Sometimes, well, they just don't. And they don't help you discover why or why not. Google Home simply rolls useless graphics as opposed to any form of meaningful status until the connections are golden end-to-end. Nyeah...that really helps