Try the steps mentioned here:
Nope, there is no option to rotate the camera. If I just physically turn the phone to landscape orientation, on DEX on my computer, the camera app is still in portrait mode, and now i'm just displayed sideways.
I apologize, this would be the only option available.
If you launch zoom on the phone screen whilst in DeX mode, it will open on the phone screen and the camera will be correctly orientated, other DeX functions still work with this setup .. but it kind of defeats the point of having DeX in the first place as you can't see the screen if using the rear camera, and if using the front camera - the video of other participants remains on the phone screen.
Seems to be a different issue with the stock camera too, the camera is correctly orientated in landscape but the image resolution is scaled to portrait resolution.
The only software I have found so far that has correctly implemented landscape mode in DeX is Cisco Webex meetings.
So, I found a rather peculiar work around. When you open you camera app or Zoom application, on Dex, go to settings and select "Update device". This will trigger the device to open the update screen on the phone and make the camera go to landscape. The phone I used was an unlocked S20.
I see that this does rotate the camera app from portrait (stretched) to landscape (normal), but we need portrait normal in landscape. And, this has no effect in the Zoom app.
It seems to be a compatibility between Zoom and Dex. Like Dex takes control of the camera hardware and only feeds portrait to Zoom.