Ubuntu 18.04 (and possibly later versions) switched from LightDM to GDM3. Connecting to the login screen with VNC while using GDM3 is currently not possible. The easiest way to get this VNC functionality back is to simply switch back from GDM3 to LightDM (which is still being activity developed and used by many Linux distros).

Install LightDM

apt install lightdm

Should you for some reason come to regret switching to LightDM: