For me, the easiest method to configure monitor under KDE is by installing KScreen from repo. Once installed, you can configure your monitor setting through Configure your Desktop)>>Hardware>>Display and Monitor> Display Configuration.
You may never need to configure it at all since a lot of time I’ve found KScreen smart enough to auto-configure my monitor. Still not sure why KDE developers don’t make KScreen as the default screen management software. In my opinion it’s much better than the default one.
Image credit: Techverse