You are right, I was thinking that DisplayPort on my monitor does support data transfer, but after research I see, that this is not the case.
I was also struggling to fix the resolution issue using BetterDummy on the 4K monitor, but no luck with that. Also if I create 2 dummies for both external monitors the outcome is the same as without BetterDummy. QHD monitor is working fine at 2560x1440, but the 4K is a bit blurry. Canīt work like that