I found a prior Q&A (that is now closed preventing me from replying) that says this:
So I want to allow my machine behind NAT to be accessible via FTP.....
The answer posted by a user was this (my emphasis):
Yes, portforwarding should work, provided your router firewall is FTP aware, as there is a second connection established on a random port with ftp, which needs to be opened and forwarded automatically by the router.
I have installed QuickFTP on my Mac Studio. It works well from my local network but fails from outside the network. I have forwarded the port on my router. However, I am not sure what is meant by "there is a second connection established on a random port with ftp which needs to be opened and forwarded..."
I am currently using port 2121 for both the external and internal ports and as the listening port for QuickFTP. Should I instead port forward a range of port numbers? Or, is there a separate router firewall setting that I should change?