There are two types of Atmos for home theaters: Dolby Digital Plus + Atmos (DDP Atmos) and TrueHD + Atmos (THD Atmos).
Streaming services such as Netflix use DDP Atmos.
Movies from Blu-ray discs use THD Atmos.
Amazon FireTV devices support DDP Atmos. They do not support THD Atmos.
If you want THD Atmos, you will need a Nvidia Shield Pro, connected directly to your Denon.
Shield Pro ← HDMI → Denon ← HDMI → TV
Note that the non-Pro (tube) Shield has problems with high bit rate videos such as 4k HDR movies with TrueHD audio.
See this thread for additional details:
[INFO] Plex, 4k, transcoding, and you - aka the rules of 4k
