Unable to connect Apple TV to Wi-Fi network

It happens only with the router at my main house which is a wifi 6 router, it connects to the router fine at my other house which is a wifi 5 router.

That is an important clue.

I think the issue might be in WPA3. WPA3 would be too new for a 2012 device. Try if you have a WPA3+WPA2 compatibility mode available in the router.

Wi-Fi Protected Access - Wikipedia

Protocol security - Apple Support

Recommended settings for Wi-Fi routers and access points - Apple Support

Try the following:

The 30-30-30 reset. There are no specific instructions for this model yet, but you can always do the 30-30-30 reset as follows:When router is powered on, press the reset button and hold for 30 seconds. (Use a paper clip or something sharp)While holding the reset button pressed, unplug the power of the router and hold the reset button for 30 secondsWhile still holding the reset button down, turn on the power to the unit again and hold for another 30 seconds. (Source: https://www.router-reset.com/info/Apple/TV-A1378-2rd-generation)

Then try to connect to the network, the same error (-100) will come, but than it will ask for the wifi password.

Another important thing is to change the Wifi Security mode to WPA2 PSK. Now it works for me.

Identify your Apple TV model - Apple Support

A1378 would be 2nd generation. A1427 or A1469 would be 3rd generation.

Try selecting for Other network, then enter both the network name manually, and the password.

Unless someone else chimes in with a new good idea, I would suggest to contact an expert from the the Apple Support staff for further assistance. Contact Apple for support and service (phone numbers) or start your support request online (chat/​email/​callback).

Thanks for ideas. Have a 2nd generation Apple TV. Put in a Google wifi mesh system yesterday and changed my network name and password. Had to put in the new password for all of my apple devices. No problem except the Apple TV which wouldn't ask me for a new password. After restarting everything several times and spending time doing the same things with apple support I was about ready to give. Remembered that the google wifi setting was WPA3. Turned off WPA3 and restarted everything and still no luck. When I went to "Other" and input my network name it asked me for a password. Success! Yesterday I also wasted 5 hours trying to set up Google wifi mesh with Google Home on my iphone with no success (and fail by Google help desk who said I should return the hardware). Found a similar help post that said that the setup only worked with Google Home on Android devices. Found my wife's old Android phone, charged it up and set it up the mesh easily with Google Home. Patience is definite a virtue with this stuff, at least for me. Thanks for the ideas!

Another device rendered useless by Apple to force us to buy a new one.

I use my older Apple TV with iPhone hotspot after the latest iPhone update I get the same error code.

Same. Couldn’t work out what the issue is but definitely since iOS 15 update.

I also have the same problem, apple tv 2gen and iphone ios 15.

Exactly the same issue for me. Apple TV 2gen. And all happened immediately off the back of the iOS 15 update.

Come on Apple - this wouldn't be another form of forced redundancy would it!??