I have a Google/Nest Learning Thermostat gen 3 and the addition of it to the Nest app consistently fails on the final step. WiFi signal is strong at the thermostat, it is logged in to my home network, both the device and Nest app on an Android phone are up to date software. I can see the device and its IP address on my home network. There is a "C" wire and battery is strong. The router/firewall are not blocking the associated ports (80, 443, 9543).
Google support asked me to contact my ISP and confirm that it is supported, i.e., no ports are being blocked or blackholed (I assume).