Display port connection

I am still new to Qubes OS and struggling a bit with my setup.
So far, I have connected my desktop computer to my display through HDMI using a PCIe 3.0 graphic card.
Now I like to use my display port connection with the display (HDMI does work).
Whenever I plug my connection into the display port, Qubes starts, but then ends up with a black screen
so I can not even pass the password step.
The display port connection is working with a different OS, so it´s not the problem of the display port or graphic card.
The resolution of my screen is: 3440x1440

I have read: Video RAM adjustment for high-resolution displays
“Caution: This page is intended for advanced users.” - which I am not…
GUI Configuration | Qubes OS,
but I am not sure, if this relates also to external PCIe graphic cards or not, but I tried…

value for the min RAM calculated: (3440x1440*4/1024)=19350, so lets say 22000…
so I typed into the terminal in dom0:

qvm-features dom0 gui-videoram-min 22000
restarted my comp

But no changes, still the display port connection does not work.

Can anyone please be so kind and give me a hint on what to do, so I can use the display port connection.

Thanks in advance!