I'm sorry to hear that you lost access to your thermostat in the Google Home App after changing your Wi-Fi network. I'd love to help.
A change in Wi-Fi connection will disconnect your Nest Thermostat from the network. To isolate, were you able to reconnect your thermostat to your new Wi-Fi network? Can you still access it in the Nest app? Also, please make sure to login with the same email address in the Google Home app associated with the Nest app on your thermostat. If so, check for other available structures where your thermostat might be connected. Check out this guide on how to switch homes.
Please let me know if that helps, or if the issue persists.
Just one quick final check in here since activity has slowed down. We'll be locking the thread in the next 24 hours, but if you still need help, I would be happy to keep it open. If there's more we can do, just let me know.