I cannot reconnect chrome cast to my wifi. Says check my att network

All you need to do is just Download Chromecast app from the Google Play store. Unfortunately, the Chromecast doesn't support 5 GHz WI-Fi and is stuck on the 2.4GHz band; the 2.4GHz band is quite congested with traffic (especially if you reside in a huge apartment complex where there maybe you're surrounded by dozens of routers). Rest those who are new to this streaming device can get the Google Chromecast Download by stepping ahead to the Google Play Store.

To change the WiFi network your Chromecast is connected to, you will need to perform an FDR (Factory Data Reset) on your Chromecast and go through set up again.

There are two ways to Factory Data Reset (FDR) your Chromecast:

Factory Data Reset your Chromecast from the Chromecast app. You will find the option to FDR under 'Settings' or 'Menu' or
Physically hold down the button on your Chromecast for at least 25 seconds or until the solid light begins flashing.