It looks like it doesn't actually support changing the port currently, despite the documentation saying it is possible. I already use port 8080 so kind of stuck until I can use a different port.
I got it to run on a different port just fine with the MO_PORT environment variable, FWIW.
Turns out its just too early in the day. I wasn't saving the variable beyond setting it. So when I switched terminals it didn't exist. Put it in my bash profile and all is well.
You can use .env file too, or even put it before the command like that: