So if it's plugged direct to the 9.1 I can only think:
1. It's not a 4k fire stick. I mention this because I bought the wrong generation of fire tv cube 2nd hand and had to sell it on.
2. Something else in the fire tv settings isn't correct. I'd try one of the threads for streaming devices.
3.You don't have a Top tier netflix account
4. The bar is knackered.
I understand your fustration but you need to start at the source and work your way through it
It 100 percent is a 4k. Unless they shipped the wrong model.
I have the middle tier netflix.
I was reading some posts that the "new" 2020 fire stick while only 1080p is the same as the 4k internally yet it's running 7.x firmware while the 4k is running 6.x.... Apparently the new model has better audio capability according to some of the Q&A on the Amazon store
Also some discussion that the 2020 FS can DECODE atmos while the 4k is only able to PASS THROUGH atmos? Maybe that's due to the newer firmware?
Could that be the issue? Can the 9.1 decode atmos, or must it be fed a decoded signal?
Lastly, I never upgraded the bar's firmware by connecting it to wifi or ethernet (can't remember if it has a cat5 port). Reason being that some complaints were registered here after updating. Decided to steer clear until the next release and see how it fares. Could that be an issue? I can't find a change log for the updates