My installation experience was successful but convoluted
 

2019 Jun 30
Haven't been able to use a second monitor for almost a year but have full functionality again on macOS. Had to follow an odd process to get it to work and wanted to share the experience in case it may help someone else.

Machine: mid-2012 MacBook Pro
System: macOS Mojave 10.14.5
Monitor 1: ASUS VP239H-P connected via thunderbolt cable
Monitor 2: ASUS VP239H-P connected via USB through Diamond Multimedia BVU195.

Downloaded and installed the DisplayLink macOS 5.1.1 update for Mojave 10.14.

Followed the steps in the article macOS 10.13 of 10.14: Video functionality not enabled to enable the DisplayLink software.

This pathway follows:
System Preferences > Security & Privacy > General tab > click on Allow button to enable DisplayLink software

After several restarts and various methods, I had to uninstall the driver and then reinstall it. The software did not need to be enabled again.

Finally, the key to getting the both monitors functioning on machine is the order of connection. Connecting the thunderbolt-connected monitor first excludes the functionality of the USB-connected monitor. The USB receives a signal, but the monitor flashes between a black screen and a blank screen. Additionally, using the Detect Displays option in the Displays manager does not find the USB-connected monitor. Connecting the USB first identifies that monitor as the primary, but still allows the thunderbolt connection to function. If you want the thunderbolt connection to be the primary in an extended screen, you will need to adjust it as described in the article How to configure DisplayLink displays on Mac OSX.

Good luck!