First, hard restart everything (unplug, wait 30 seconds and plug back in.).
Then check the following:
*settings on the device
*type of HDMI cable that you are using
*Settings on the TV
Mine was set up right out of the box and I was able to watch 4K Blu-Rays immediately. To be fair, I haven’t tried a normal BR or DVD.
Check those three things and try to update the devices firmware if you can. That should fix the problem.