Found this guide to add custom resolution and refresh rate when using Wayland/Gnome and this should work on KDE too:
In short you have to add a video value to GRUB_CMDLINE_LINUX_DEFAULT parameter in /etc/default/grub as root:
video=<port-name>:<resolution-w>x<resolution-h>@<refresh-rate> #format
After editing you have to update the grub entry and reboot:
sudo update-grub