I have recently bought a new Samsung Q60T UHD 4K tv, on the Knowledge that my new Samsung s20 ultra can use "Samsung Dex" mode, wirelessly, on the TV. When I engage Samsung dex wirelessly from my s20, the correct TV appears in the list on my phone, and I select it.
When Dex starts on the TV, it is extremely pixilated, low resolution and laggy. I am connected over 5ghz WiFi.
Conversely, when I use wireless screen mirroring from my s20 to the TV, the image is perfect and it works very well, so the issue is just for Dex mode.
I have found a "fix", but I believe there is a software bug.
The "fix" is to go into the smart things app on the s20, select my TV, and select "Play phone Audio on TV". This then displays a music casting screen on the TV, where the TV asks me to start playing music on my phone. Once on this screen on my TV, I can then select Samsung Dex from my phone, begin the process again, and this time, Samsung dex works perfectly, wirelessly on the TV.
This is the only way to get Samsung Dex to work on the TV wirelessly, to go through the smart things app, and through the audio casting screen.
I hope this helps you solve this issue so I can use wireless dex normally.
If there are any settings I should change on either the TV or the phone, please advise.