Getting dolby 5.1 surround with FCPx and Compressor

I have spent 2 days researching and experimenting with ways to get a surround sound project from Final Cut ProX to a mov, m4v or mp4 video file and have AppleTV 4K sense that and play it correctly through my Denon receiver and home theatre system. All the instructions on the Internet have proved unsuccessful - although I haven't tried Handbrake yet.

The only way I have been able to get the dolby surround sound to work as programmed in FCX is by burning a Blu-ray disc. Then it works as expected.

I have even tried exporting an AC3 audio file and then I have tried using Subler to import that audio file with 6 channels and export to both mp4 and m4v without success. The files export fine but on the Apple TV it reverts to Matrix mode, so the surround sound is not playing expected. I have tried almost all possible permutations and combinations of doing this.

I have a couple of a Dolby Atmos mp4 files that I downloaded off the Internet and those work through Apple TV and the Denon receiver so I assume that the problem is in how my own videos are getting encoded.

In Compressor if you select Compressing for Apple Devices, it has an option "Include Dolby 5.1 Audio Track: Select the checkbox to add surround sound as a Dolby Digital audio track for playback on Apple TV".

I checked that option and nothing happens. Does anyone know how this is suppose to work?

Surely Compressor can do this without relying on some other app like Handbrake or Subler since it can do it properly for a blu-ray disc. I don't get it, why is this so difficult to do? All I read on the Internet is frustration and highly technical workarounds by many video creators over the past 10 years on this matter.