The Samsung Galaxy S9+ supports DisplayPort 1.2 over its USB Type-C connector but unfortunately the phone doesn't work with the ASUS MB169C+ USB Type-C Portable Monitor supporting DisplayPort.
After we connected the Samsung Galaxy S9+ to the ASUS MB169C+ USB Type-C Portable Monitor using the USB 3.1 USB Type-C to Type-C cable that came with the monitor, we weren't able to see any image appear on the monitor from the Samsung phone.
Apparently there is some problem with the DisplayPort output in the Samsung Galaxy S9+ working with the DisplayPort input of the ASUS MB169C+ USB Type-C Portable Monitor.