Apple TV keeps saying my WiFi password is incorrect

I had this problem with my Apple TV 4K box right after my DSL modem was replaced with a new one. I figured out how to fix this while troubleshooting. In the modem settings, when I disabled the 5GHz wireless radio, devices must now connect via the 2.4GHz wireless radio. The Apple TV 4K now accepted the password and connected successfully to my WiFi network. When I disabled 2.4GHz, and enabled 5GHz the Apple TV 4K gets the "incorrect password" issue again. It seems when both 2.4GHz and 5GHz are enabled (the default setting), the Apple TV 4K was connecting via 5GHz (and getting the issue). This is really only a workaround until Apple fixes this issue with 5GHz connections and updates the software.

1. If your Apple TV can't connect to Wi-Fi - Apple Support
2. Try the ‘Other…’ option instead of selecting a listed discovered network. Especially if the password has been changed.
3. For Apple TV (4th generation or earlier), verify that the router or access point isn’t set to use WPA3, as only device models from ~2018 or later support that. Instead use ‘WPA2/WPA3 Transitional’.

Sorry to hear that. At this time, I would suggest to contact an expert from the Apple Support staff for further assistance. Contact Apple for support and service (phone numbers) or start your support request online (chat/​email/​callback).