I bought a new Samsung Q80T, I previously had a TCL that I would connect wirelessly too via WINDOWS+P and use it as a third monitor. This worked without any issues for many years.
My new TV it works for about a day, and stops working. Also SmartView only works for the same amount of time and that stops working as well, Samsung DeX has the same issue. My TV is connected to the network via ethernet, my phone is on Wifi6. I have the Samsung Galaxy Note 20 Ultra, the computer, phone and TV are all on the same layer 2 domain.
When Mirror Casting no longer works I can still ping the TV but nothing can connect to it. I have the TCL still, and all devices work with that still.
To fix the issue I just have to unplug the Samsung TV, or hold the power button on the remote. Once the TV reboots it will work for a day or so, before it quits working entirely again.
Samsung support says my network must be the issue. I don't think they are correct.