I have Dell G3 15 and I have installed Ubuntu 18.04 and at first my second monitor was not working.
And after I performed this tutorial: http://ae0bq.org/?p=137
At this point only my second hdmi connected display seems to work. My laptop display remains black.
I aslo tried switching from gdm3 and lightdm with the same output. Under wayland, on gdm3 I was able to work on my laptop display but not on the hdmi connected one.
How can I make both displays work? (there is no hardware failure)