What is HDMI Audio Output set to on the Xfinity box?

Expert mode.

Try the Auto Detect setting.

I almost included that I have tried that multiple times.  Many multiple times.  But thanks for making sure I had explored all options with the audio setting available on the XG1v4 box.

It could just be that the channels you are watching don’t support 5.1 audio but only stereo. Are you just watching cable channels? What are your TV’s audio settings set to?

Before posting, I checked all of the premium channels, with the same negative result.  For my TV, a Samsung, it plays Dolby Digital just fine when I pull up content from one of its apps, such as Netflix, Amazon Prime, Hulu, etc.  (Within the TV’s audio settings, I also have Expert selected but when the source is the Xfinity box, the only options for audio input and audio output are PCM.  The Dolby Digital options, for audio output, are grayed out.

Are there any streaming apps on the Xfinity box like Amazon Prime Video? Do you get stereo from those apps too?

Yes, multiple apps via the Xfinity box, and they all also show only PCM stereo.

One thing you could try is swapping the HDMI inputs for your AppleTV and Xfinity DVR.  Sometimes, TVs have different specs on different inputs.  If your AppleTV works and DVR does not...then your DVR isn’t giving you DD+.  If your DVR starts working and the AppleTV doesn’t, then it’s the HDMI input.

Try connecting the Xfinity box using the HDMI cable and HDMI input you are currently using for the Apple TV 4K and test a 5.1 movie on Prime Video.