I made a similar post just now, I am using a different soundbar and only have the issue in Disney+ currently. Atmos works OK for me in Netflix so you should be able to get this to work. Did you check whether your TV supports eARC? I think you need this to enable atmos and as you mention you have a 2018 model it might be too dated to support atmos pass-through.
In a way it feels a bit of a waste of money to purchase an additional media player after spending a large amount of money on a TV that should have smart functionality, but if there are no other ways to solve it I'll look into it as well. Indeed I'm lacking a port as well due to a few game consoles and the soundbar wasting one port. Could you tell me which hdmi splitter you're getting that supports atmos & 4k pass through?
