4k UHD TV all of a sudden pixelated / grainy?

It started two days ago while watching Breaking Bad on Netflix. At first I thought it was Netflix or the show itself, but when I wanted to watch a movie on Disney # in UHD today; the same. The picture is as pixelated / grainy as on an old SD TV. Was looking for updates to the TV, nothing; Also uninstalling the Disney + app and restarting it does not bring anything, just like restarting the router. Would it help a bit to reset the TV to the factory settings or is something like that considered a defect that is sufficient for an exchange (TV is from TCL, approx. 6 months old and was bought in Media Markt)?

Have you ever tried a DVD? Maybe it could be because of the internet connection

Not yet, but I will.

It's the same for a Blu-Ray disc over PS4.

Then I would try an exchange or if necessary try to restore the standard settings on the television. Does everything run through the ps4 for you? Maybe it's because of her