The UP2715K does not have a single port that can handle the 5K bandwidth. It was one of the first 5K monitors and required the use of two DisplayPort (DP 1.2) inputs simultaneously a method known as MST.Timeline of 5k computer screens
I wonder which company was first to develop the 5k screen technology? Maybe Dell?
For many years and particularly with Thunderbolt 2 (Mini-DisplayPort) macOS drivers did not natively support the MST configuration required for 5K on this specific monitor. This often resulted in flickering, blank screens or only working at lower resolutions (like 4K or 2560x1440).