Regarding the issue described here, the following was posted today at http://www.displaylink.com/downloads/macos .
DisplayLink has worked for many months to maintain support for its users during the transition from macOS High Sierra to Mojave. While functionality such as Ethernet and audio or charging, where implemented, is unaffected on all releases, display connectivity has been limited on macOS releases from 10.13.4 through 10.13.6.
Developers/users with access to Apple’s fifth developer preview of 10.14 Mojave, released on 30th July, will see full support has now been restored for multiple extended DisplayLink displays where DisplayLink driver v4.3 beta 5 is installed. We are doing wide scale testing of v4.3 beta against this preview and will release a new driver upon general availability of 10.14.
Users of 10.13.4-10.13.6 should also use v4.3 beta 5 as it contains a workaround to enable one extended USB display (720p or 1080p). An FAQ details how to enable Apple AirPlay with DisplayLink display for extend or mirror mode.
Users on macOS 10.13.3 and earlier should remain for the time being with DisplayLink driver 4.1 to maintain full extended display functionality.