I purchased an Apple TV 4th Generation for a Samsung HD TV (720p) I have in my bedroom. Yesterday I watched the first movie on it ("mother!") which I rented from iTunes movies. I really was expecting a better quality resolution at least like the one I get on my cable TV provider's HD channels, but no... I just found it to be too grainy, almost like standard. Maybe something's wrong with the settings? I didn't change anything, it's all default. Or was it the movie itself? I will watch Netflix and Prime Video to compare later on today.
TV connected directly to your TV or going through something else that might be using SD settings? Is the connections via HDMI cables or something else?