I am facing the EXACT same issue with my QN800A and Q70T. From all my research, it looks like I'm going to have to buy a Google Chrome and connect it to the Q70T to get atmos to work. Ridiculous! This is juat as bad as the YouTube app not showing 8K content. I wish Samsung would smarten up and fix all these ridiculous issues.
Finally got Atmos to work on Samsung TV with in-built Netflix app with my Samsung Q90R Soundbar. First step, you will need to go to Settings > Sound > Expert Settings. Change HDMI-eARC Mode to Auto. Digital Output Audio Format to Auto.
Well this will not change Netflix compatible movies to Atmos yet. You'll need to go to Apps, look for the Netflix app, hold the OK button on your remote for like 3secs. A pop up box will appear. Click on "Reinstall".
Once done, go back into Netflix app and check out those movies with Atmos compatible.
This has worked for me. Hope it will for you.