The USB Ethernet on the UD-CAM didn’t show up right away on my 2018 MacBook Pro running 10.14.3. This is due to System Integrity Protection not playing nice with the Realtek NIC driver.
To fix it, restart your machine while holding Cmd-R to bring up the recovery menu. From there, select the Terminal from the Utilities menu. Enter csrutil disable and restart the system. It should be working right away once you’re back into the OS.
Bear in mind you do lose some security by disabling SIP; the NIC driver not loading is by design with that–the system doesn’t know it can be trusted. What we’re doing here is telling the machine to allow unknown drivers to run anyway.