You probably have to put your modem/ ISP equipment in bridge mode if it isn't already, else you'll have double NAT.
And since iOS is mentioned:
Make sure the device used for the setup process with the Google Home app have no VPN enabled/ connected; not any 3rd party product, nor the "Apple iCloud Private Relay" ( https://support.apple.com/en-us/HT212614 ).
Make sure the Google Home app has been granted permission for "local network access" ( https://support.google.com/chromecast/answer/7172427?hl=en ).
Make sure that proxy is set to disabled while connected to the Nest Wifi network ( https://www.expressvpn.com/support/troubleshooting/disable-proxy-safari/#ios ).
Awesome — glad to hear that and thanks for getting back to us. It looks like we can consider this one complete, so I will lock the thread shortly unless I can help out with anything else.