I’m using EndeavourOS with KDE.
The display is correctly oriented when logged in but it doesn’t rotate correctly when I’m logged out.
EDIT: corrected the post. This happens when logged out, locking the screen has it displayed correctly.
Put one hand on the top and one on bottom and rotate the screen by 90° or π/2 radians
If you have to read PDFs regularly having a vertical monitor is a lifesaver.
On the arch wiki for SSDM I found this:
Changes to your display configuration made in a Plasma Wayland session (e.g. monitor layout, resolution, etc) will not persist to SDDM. To make them persist open Plasma's System Settings and navigate to Startup and Shutdown> Login Screen (SDDM) and click "Apply Plasma Settings...". You will need to have permission to perform this action.
You should give that a try
I can't remember where I found it, but here it is. I thought it might be from here (I've used these previously) but it is not.
usually monitors can be freely rotated. if yours can't, the back usually has a square vesa mount on the back and you can just take out the four screws and reattach it the way you like.
I've just been logging in upside down for a couple years. My monitor's vesa Mount is like 3 inches from the top for some reason so having it upside down is the only way I can get a reasonable ergonomic height
Which display manager are you using?