I have this problem too for my wired connection only.
It is managed by Network Manager and yet it doesn't appear on the the Network List.
If I connect to a Wifi network that will appear on the list and allow me to toggle on/off.
Ubuntu 17.10 (not headless server edition!)
vino 3.8.1-0ubuntu12
gnome 3.26
I have this problem too for my wired connection only.
It is managed by Network Manager and yet it doesn't appear on the the Network List.
If I connect to a Wifi network that will appear on the list and allow me to toggle on/off.
Ubuntu 17.10 (not headless server edition!)
vino 3.8.1-0ubuntu12
gnome 3.26