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)
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.