Hi there!
My Mi Box S has WiFi connection and apps to test wifi quality shows good signal and speed but when I open youtube or prime video it won't upload the video.
I think I'll do a factory reset but I want to know if it could be an issue of the system.
Edit: Factory reset solved the issue.
The wifi chip on the Mibox is crap. Get an USB to ethernet adapter and use that instead. I used to use wifi initially but ended up using ethernet myself and it's much better than before.