Thanks for reaching out. To confirm, is your Chromecast won't connect to the network? Please reboot your Wi-Fi router then, manually connect to the Chromecast SSID. Here's how: Go to the phone settings > Open Wi-Fi and connect to the Chromecast SSID (Example: Chromecast1234, Living Room TV, Bedroom TV, etc.) > Go to the Google Home app and try to set it up again.
If the issue persists, factory reset your Chromecast
Just checking in to make sure that you've seen my response. Please let me know if you have other questions or concerns as I will be locking this in 24 hours if I won't hear back from you again. Feel free to start a new thread and we'll be happy to help.