I have a Chromecast and a UHD Chromecast - both can (and are) hardwired when setup.
Yet I have simply stopped using them and rely solely on Airplay instead.
They just need to spend more time on C/Cast...
What I found was to successfully reset the CCu, you cannot leave any existing devices turned on, weird. Its as though I turn the phone off but it sees that tablet so it wont go back to factory.