When I upgraded to Mojave, allofasudden I could not see the serial port of my Nano … it wasn’t showing up in /dev at all. So I did the usual … downloaded the latest CH340 drivers, then removed the .kext files from /Library etc. and re-installed the drivers and still it would not work.
This is what fixed it:
sudo su <enter> (enter your password) rm -rf /Library/Extensions/usbserial.kext
DO NOT INSTALL ANY DRIVERS … it should work fine after the reboot.