Stouchi - 8K HDMI 2.1 Cable 6ft 48Gbps - 6ft - $12 - Amazon.

I had a similar issue. One thing is make sure all your HDMI cables are certified. I use these HDMI cables.

Also, check if your avr has an "enhanced" 4k mode. In settings on my Denon there was a 4k standard and enhanced under HDMI settings.

Also, check in your display settings on the shield if you're using 4k/60 or 4k/59.9. I've read Disney+ is picky about that