Hello netsprite, I don't use the night mode but I tested it for you (out of interest). Night mode works for me. There is the possibility to activate the night mode directly --> Settings > System > Display > Settings for the night mode - button "Activate now". If it is greyed out and cannot be operated, try updating the display driver or reinstalling it. Also check, as already mentioned, if your external monitor supports this.
I wish you much success in trying it out.

Hello netsprite, I'm sorry I can't answer your question completely because I don't use the night mode myself. I can only imagine that the external screen itself must have this function so that it can display the changed tonal values. Maybe you also need a different driver?

In any case, I wish you success in your search.