How to set my monitor as my default display?

Yes.

Where is “applications/configure desktop/Display Configuration” exactly? Which desktop manager are you using? I don’t seem to have this option (I use cinnamon).

That’s for KDE. (“Configure Desktop” is KDE’s configuration application, also known as “Systemsettings”)

I don’t know Cinnamon, but as it is a fork of GNOME, it should have something similar as well in its control center (or whatever it’s called in Cinnamon).
But please bear in mind, that this only applies to the user session anyway. It has absolutely no effect to the login screen.