A:Answer Dynex support said here that it supports 4K, but it actually does not. I found verification on the Dynex site that it doesn't do 4K, only up to 1920 x 1200.
https://www.dynexproducts.com/pdp/DX-PD06512/8720139
I was able to do 4K with a "Rankie Mini DisplayPort (Mini DP) to HDMI Cable, 4K Ready, 6 Feet".