Companion Bluetooth/WiFi Issue

I am using an Apple 4k TV which is connected via ethernet and the companion app on iPhone 11. I sometimes have wifi signal issues with the phone so have connected phone onto same ethernet as Apple TV but it will not now connect the trainer etc to the game. Does the app need to be on wifi!! and if not what is wrong here. As soon as I unplug ethernet from phone and turn wifi back on it connects. Any suggestions as I thought the app connection was via Bluetooth to the Apple TV and not wifi.