Why can't I use this monitor full resolution at 60hz on my 2013 Mac Pro with OS X Mojave?
In order to drive a 4k display at 60 Hz over HDMI, you'll need a GPU supporting HDMI 2.0. The 2013 Mac Pro supports HDMI 1.4 by default.

To get this working, you'll want to use a cable plugged into the DisplayPort connector on the monitor, and into the Thunderbolt 2 ports on the Mac Pro.