ATV1 not connecting to WIFI error -100

If your Apple TV is a 3.9" black box, then it is at least 2nd generation.

Identify your Apple TV model - Apple Support

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. 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.