The same notebook, the same monitor, just different cable.
HDMI: No QHD possible.
2048x1536, 2560x1600 (= bad aspec ratio), 4k (@ 30 Hz).
DP: 2560x1440 QHD on the list (@ 60 Hz runs fine)
The P2715Q HDMI 1.4 port can do 3840x2160 30Hz and 2560x1440 60Hz. What specific laptop model? What operating system? Have you tried testing the P2715Q on a different computer with HDMI out?
Sorry, it was a reaction to another message. Should not be standalone here.
Neverheless: XPS 9550, 6th gen i7, 4K with Dell P2715Q display
I do not expect 4K @ 60 Hz. I wanted QHD @ 60 Hz, but the HDMI connection does not offer this resolution, see my post above.
The same things, just connected with a DP caple and QHD is there to choose.
The P2715Q HDMI 1.4 port should do 2560x1440 60Hz IF the laptop GPU and driver support it. If not, try creating a custom resolutions using 50Hz. Or try 2048x1152 60Hz.