Silly question, did you enable both settings in tvOS ?
Great news! However, I just installed on my Apple TV 4K, and 5.1 isn't working. Everything is playing in 2.0. The picture looks great though. My other apps work fine, Netflix and Hula (to name a few) all play in 5.1. Anyone else having this issue?
Found my problem, I was using Safari changed to FireFox and its working.