Ubuntu 20.04 login screen doesn't appear on the primary screen
Solution 1:
I have fixed the problem with these line of code:
sudo cp ~/.config/monitors.xml ~gdm/.config/monitors.xml
It only works if the desired monitor has been marked as primary first.
Solution 2:
I had to do some additional steps:
If you haven't done it yet, go to Settings → Devices → Displays and set your primary monitor. Save your settings.
Open a terminal and run the following:
sudo cp ~/.config/monitors.xml ~gdm/.config/monitors.xml
sudo chown gdm:gdm ~gdm/.config/monitors.xml
sudoedit /etc/gdm3/custom.conf
In the file uncomment WaylandEnable=false, and save.