I have a spare machine with a broken screen and I could access it with the following configuration:
# Remote desktop
services.xrdp.enable = true;
services.xrdp.defaultWindowManager = "${pkgs.gnome.gnome-session}/bin/gnome-session";
services.xrdp.openFirewall = true;
# Disable the GNOME3/GDM auto-suspend feature that cannot be disabled in GUI!
# If no user is logged in, the machine will power down after 20 minutes.
systemd.targets.sleep.enable = false;
systemd.targets.suspend.enable = false;
systemd.targets.hibernate.enable = false;
systemd.targets.hybrid-sleep.enable = false;
Without the second part, the machine was auto-suspending even though I was remotely connected. I could be remembering wrong, though, as it`s been a while since I last used this.