Can you see the menu if you press the home button? If you can go to settings and change the screen resolution fro 2160 to 1080 or 1080 to 720. Im assuming you have rebooted the box, and moved to a different HDMI port. Checked you are on the correct source(input) Try rebooting the actual TV set from the mains.
Have you got a different TV you can test if it’s the box?
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