Hello StefanSalewski, many thanks for your reply!
I have my laptop with a builtin 15" 3840 x 2160 (16:9) resolution, and my external monitor, a 32" 3440 x 1440 (21:9) resolution. The monitor is connected with HDMI.
My question is how I can force GNOME to set a custom resolution, which I know works with X11, when running Wayland.