Which Denon do you have? I’ve got the X3400 and it definitely decodes Atmos with a 3.1 setup. I’m not saying you get Atmos effects, it simply downmixes additional channels to 3.1.Normally its worth going for the Atmos track - the higher bit-rate does represent an increase in overall sound quality not just Atmos data. My Denon will only allow Atmos if you add in some extra speakers - but it's worth it.
But currently D+ have removed the top tier Atmos audio option even though its listed in the app and we're just left with a low bit-rate 5.1 track. Other services may be doing the same sort of thing.