How do Apple TV 4K works with AT&T TV

only way to hook up an appletv is to connect it to ac power and then connect it by hdmi to a tv which has a hdmi1.3 port or later

and in many cases one needs to use the tvs remote to set the hdmi port one connected it to as active inputs source

Set up your Apple TV - Apple Support

Once set up, download the AT&T TV app from the on-device App Store.