Upon Mac OSX High Sierra 10.13.3 just had to set up Wi-Fi hotspot with password - tap on Set-up Wi-Fi hotspot just below Portable Wi-Fi hotspot, but presumably that it what you did?
Shouldn’t make a difference but I have also enabled USB tethering on this Mac using HORNDIS:https://forum.fairphone.com/t/how-to-use-usb-tethering-with-macos-10-13-high-sierra/34407/2