How to control Apple TV volume using iPhone Mirroring?

UPDATE: I contacted Apple support on this, here's what they confirmed:

Unfortunately the only way to control the volume from your Apple TV would be with the iPhone paired as a tv remove or from the remote itself. Using the iPhone mirroring app just controls and changes how loud it plays back on your mac

But it's actually a great idea and I will share it as suggestion to my managers and tvOS software developers to implement that feature in a future update. If you go to Product Feedback - Apple you can also leave it as a suggestion.

I figured this was the case, but Apple support confirmed it. I left product feedback as suggested. I mean, afterall it would be awesome if those volume buttons were functional through the mirroring app on macOS. Otherwise it's just confusing, at least for me anyway, to have them there along with the rest of the buttons that are actually functional.

Yes, in my case, the Apple TV Remote function on my iPhone works great from the phone itself, and I have it connected via HDMI-CEC, as previously stated—no issues with the connection or volume control when using my iPhone in conjunction with the Remote app. Volume control works great. It's just non-functional via iPhone Mirroring. Unfortunately, those volume "buttons" that briefly appear for a few seconds when you first open the mirrored Remote app on macOS serve no purpose other than for looks, I guess?

Support confirmed these volume buttons aren't functional through iPhone Mirroring. You can't do anything with them. If you click on them, nothing happens. They fade away after a second or two and do not reappear unless you close out of iPhone Mirroring and reopen it again - it will once again display the volume up and down buttons on the left, along with the non-functional Mic icon to the right. These functions will only work when used with the physical remote or the Remote app on iPhone.

Hello, thanks for the reply. To confirm, my physical Apple TV remote works just fine controlling the TV volume through Apple TV, and the same goes with my iPhone, I can control the TV volume just fine via the Remote app. My question is, when using iPhone Mirroring on my MacBook, and I open the mirrored Apple TV Remote app, the volume buttons appear briefly on the left (along with the mic icon on the right) and then it will fade away after a second or two.

Is it possible to use the volume buttons (the mirrored version on macOS) in the Remote App to adjust the volume of my TV? And if so, how do you get the volume buttons to reappear on the left?