No signal on a different Monitor // GTX 1080 TI

Apr 23, 2019
ASUS ws z390 Pro
Intel i9-9900K
GTX 1080 TI
Hello forum,
I am new to the hackintosh-univers, and a bit lost after years of mac-usage (plug and play) Think you can feel me! :D
Hope someone can help me out of the problem that I've got. I switched the monitor (HP) of my computer and
had no signal on the new one (LG). The first one was connected via DVI to my GTX 1080 TI. The LG got a
DP to DP connection. Here is the strange thing: when I plug the LG while I use the HP, I can see my system on the LG.
But after I disconnect the HP and reboot the computer, there is no signal on the LG, as if there is something saved from the
HP. Is there a possibilty to reset something, so that the computer/card is recognizing the LG?

Would be very happy if some could help. And please excuse my german version of englisch, I have to get into it again, also.

Latest High Sierra //