There are quite a few threads about this. 
 

Here is a cut and paste about some data:

There is the HD Fury Arcana, the Feintech VAX04101k, and the latest, is the OREI HDA-935. You can also order from those links from the companies directly, or check your local retailers for availability. Amazon may carry some, depending on location.
 

Note that there is now a HDFury Arcana 2, and likely some other providers of similar hardware that extract the audio signal before it reaches your TV. 

@Thor-Egil What exact model TV do you have?

I have a Sony KD-65XG8096.

Ah, there is a point. There are two kinds of Atmos signal, one that can be carried on an ARC connection via Dolby Digital Plus, and another (uncompressed) that requires an eARC connection. 
 

I’m not a Netflix subscriber, but I’m fairly confident they do the former, not the latter, so you wouldn’t need to get the external eARC device such as the HDFury, assuming that TV can pass a Dolby Digital Plus signal. Apple TV, on the other hand, uses MAT, which contains the uncompressed version of Atmos, and would most often require an external device. 

There are a few TV models without eARC that can play Dolby Atmos audio from an Apple TV 4K. Try these settings on your TV and test an Atmos film on Netflix on the Apple TV:

Digital Audio Out: Auto 1

Dolby Digital Plus Output: Dolby Digital Plus

Pass Through Mode: Auto

Thanks for your answers! I have ordered an Arcana-adapter, hopefully this will be a good solution :-)