A check for YouTube
Pick a public control video with no sign-in required. If the control plays on the TV and the target video does not, compare restrictions, selected account and playback format. If both fail on the TV, check the app and network before changing anything on your phone.
Check on the intended screen
Open the app catalogue on your actual device and confirm the publisher and region. Sign in only through the service or documented billing issuer.
Keep a title-level test
Use one title, episode or fixture that you can identify. Record whether the app launches, the account signs in and the item plays.