Hi ajb, I am on Mojave (10.14). I did search through the forum, but in the answers I found, the device had already been recognized as a serial port.
I did unplug and replug the device several times, but no joy. Weirdly the problem has fixed itself though! I just stepped away from the computer for half an hour and when I plugged the stick back in, it now appears as a serial port.
Thanks for your help!