Since you get 4k when you go direct to the TV those settings are not the problem.
You seem to have used the correct HDMI input on the receiver and there don't seem to be an video processing that you can turn off in the receiver that might help but take a look around in the menus of the receiver just in case I missed that.
Try a different HDMI cable. HDMI cables are device dependent so the cable that works when you connect the DTV box direct to the TV may not work when you use the receiver in between them.
Check if there are any firmware updated for the receiver that might help.
If you have to you can run the DTV box direct to the TV and use a digital audio cable to get sound to the receiver.