I just finished building an i7-8700K system using an ASUS Z370-I motherboard. I was able to install windows and everything seemed to be working with the U2718Q monitor. After getting a windows update and rebooting I saw the splash screen from the bios and then no video -- said there was no HDMI signal and the monitor went to sleep.
The monitor works fine with same HDMI cable on a Mac Mini.
Connecting a non-4k monitor that I have (not dell) to the PC using that HDMI cable shows video just fine at 1920x1080.
I tried updating the BIOS on the MB and reinstalling the ASUS built-in video driver, but no video after the splash screen.
At one point last night I was able to plug the 4k monitor into the PC and it gave me scaled video at 1920x1080 and appeared to think it was the ASUS monitor.
I haven't tried connecting via DP yet, but am wondering if others are having problems similar to this with the same or similar monitors on Windows 10. I know there are issues with Windows 10 and 4k, but this seems different from what I've seen.
Thanks.