I had 2 Chromecast devices set up, but the master bedroom device will no longer connect to the Wi-Fi. So I gained a new updated Xfinity Wifi Modem, and my first Chromecast connected no problem, but the 2nd one will not connect to Wifi. So this is 2 WiFi modems that it will not connect to, and it gives you explanation. It goes through the whole process, it even shows the confirmation code, but will not connect to the internet. The Home App can see it, and take it through the process of signing up, but it fails at the end when it tries to connect to the internet. Is it broken? It seems to work, and the app sees it, but it won't setup completely.
Thanks for posting and sorry for the late response. Let's sort this out.
A few questions: What's the LED light status in your Chromecast? How far is the Chromecast from the Wi-Fi router? Did you receive any error message? If so, what did it say?
Please reboot your Chromecast and manually connect to the Chromecast SSID. Here's how: Go to the phone’s 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 up again.
If the same issue persists, please factory reset your Chromecast.