Good evening and thank in advance for any help you may provide.
Here the (short as possible) story:

After using for almost 4 months a HP Z2 Mini with NVidia Quadro M620, connected to a Samsung UWQHD, at almost full resolution, I swapped it with an ASUS 38".

Everything stopped working, giving black screen as soon as I logged in KDE.
Tried reinstalling NVidia drivers without success, then decided to make a fresh install of CentOS 8.

Now, with Gnome only, this is the situation:
- if connected on ANY video output with DisplayPort, everything is fine, 3840x1600@75hz.
- if I connect ANY video output with HDMI, it blinks (or flicker..) at a fast rate, until I sign in. Then black screen (monitor show "NO SIGNAL")
- if I connect 1 output with DP and 1 with HDMI, both signal go through. But I cannot set the HDMI at 3840*1600, not even @30hz (consider that the same cables grant me this settings if used on a twin Z2 but with Windows10.. so cables are ok.. I think)

Any direction? I need to use it through HDMI...

I'd say 2 things:

1. Centos is a poor choice of OS for doing fancy graphics stuff
2. You need to write yourself an /etc/X11.xorg.conf.d/nn-video.conf where 'nn' is 00-99 to decide the running order. Make it 20, unless that's taken. Use the instructions in 'man xorg.conf' for the video section and google for examples if you're stuck.