My Wi-Fi changed on my Apple TV. How do I reconnect my Apple TV to Wi-Fi?

See the following Apple Support article for information on connecting Apple TV 4K to wi-fi: If your Apple TV can't connect to Wi-Fi - Apple Support

Also, ReStarting all Devices IN THIS SEQUENCE by 'pulling the Power Plug(s)

• Remove power from 'Internet Service Provider' Network Interface Device(NID), often misnamed(MoDem)
• Remove power from Router
• Remove power from AppleTV

NEXT:

• Re-Power the NID & watch Network Status Lights complete Return to Service
• Re-Power Router & watch for it's Light Return to Service
• Re-Power Apple TV & watch for 'Steady Front Light'
• These steps, IN SEQUENCE' should RESET your Working Settings & Return the Apple TV to Normal Service

Sounds like your Apple TV is 'Networked' only via WiFi.

Preferred 'Network' for you could be

'Ethernet' which requires 'Ethernet

Ports' on your Router. Costs more

too for 'Current Standard Ethernet

Cables'.

WiFi can be difficult unless your

Router setup includes Advanced

Settings and 'Router Owner Manual'

explains Security Settings.