For Atmos to work in Prime, please follow the steps below,
1. HDCP 2.2 and above
2. 4k video
3. 20Mbps or higher internet speed
4. Receiver supports Atmos
5. FireTV stick connected to Receiver
Ensure Atmos is available in the content by checking in the internet. Everyone thinks Jack Ryan is in Atmos but it is not. Tomorrow war has Atmos.
Once above criteria satisfied, you need to use hamburger menu to change audio options and Atmos will be available.
Receiver will not show Atmos but you can hear the difference if you switch between DD+ and Atmos.
Alternatively, connect FTV4k to your TV and enable ARC. In my LG TV, I set it as not passthrough but AUTO. Voila, Receiver is showing Atmos.
It worked for me!
Netflix supports only Dolby Vison and does not support Atmos due to OS is bit old in FTV4k. Netflix supports Atmos in FTV Cube.
I use Netflix in LG to get Atmos.
