Poor picture quality

I just got an Xfinity wireless cable box with my brand new LG 4K TV and when I watch Comcast/Xfinity, the picture quality is so much worse than watching shows/sports on other apps. I have played with the resolution through Comcast and that doesn’t seem to work. Looks like a lot of people have had this issue. Anyone able to solve it? Would a new HDMI cord make a difference?