1920x1080) are grayed-out in Dex settings (see attached screenshot). Pretty much the same is applicable to my Samsung Galaxy S9 with only 1 exception - HDMI resolutions above FullHD (i.e. I’ve got Plugable UD-CAM dock and it works perfectly fine with my Dell Latitude 7480 laptop and USB, Ethernet and WQHD (2560 x 1440) Dell monitor.
