Adding an Apple TV to the Home app in tvOS 11.4

Solution 1:

Was the Apple TV your home hub? If so then you can check and see if you see Homekit as Connected:

On your Apple TV, go to Settings > Accounts > iCloud and make sure that your Apple TV shows HomeKit as Connected. If you don’t see HomeKit, then your Apple TV isn't connected as a home hub.

Set up Apple TV as a home hub

1. On your iOS device or Mac, set up two-factor authentication for your Apple ID. Then go to iCloud and make sure that you have iCloud Keychain turned on.
2. On your Apple TV, go to Settings > Accounts and make sure that you're signed in to iCloud with the same Apple ID as your iOS device.
3. After you sign in to iCloud, your Apple TV automatically sets itself up as a home hub. To check the status of your home hub, go to Settings > Accounts > iCloud and look under HomeKit to see if your home hub is connected. Learn what to do if you don’t see HomeKit.