(Long title, but had to cover all the bases)
For my radio, it was `/dev/cu.usbmodem53620077003`. Trying different order of operations (plug radio first, then start JS8Call vs start JS8Call, then plug radio) may have been what I needed to get the right serial port to show up in the list. Below is a screenshot of all the settings that seem to work.
Following this post by HB9TXB Branko, I did also check the list of serial ports using this command in the terminal: `ls -lrt /dev/tty*`.