unable to log in secondary monitor

Hello,

I am in serious trouble. For some time my imac lcd screen has been problematic, blacking out increasingly. I hated the idea of buying the new one because of planned obsolescence so I decided to get an LG 21:9 external monitor instead reasoning that it is a better investment for the next system. I used the external lg to mirror the onboard lcd, but since a few days it only goes into mirror mode after logging in. I didn't notice this until the lcd started blacking out before login. Now I can't get the login screen on the external monitor and I can't log in at all.

Who can save my life?

Best, G