I did try both the kernel open source drivers and AMD’s pro edition. Most likely you aren’t using Display Port 1.2 MST, which is what the Dell UP3214Q requires

I did try both the kernel open source drivers and AMD’s pro edition. Most likely you aren’t using Display Port 1.2 MST, which is what the Dell UP3214Q requires for 60hz 4K. My testing was with vanilla Ubuntu 18.04.