I had a hard time setting up a public facing FTP connection to my Netgear ReadyNAS 104. The problem for me was the standard FTP port of 21, I’m guessing my internet provider blocks that port. I had issues with both Cox and Charter internet connections. The screenshots and settings below are from an Apple AirPort Time Capsule, Netgear ReadyNAS 104 and Panic’s Transmit for the FTP client. I am also using a static IP address.
This is the setting that finally made it work for me. I had to change the port from 21 to something else, I chose 21202. There was already a few devices using the default passive ports, so I just changed those as well, but they should be fine if nothing else on your network is using them (AirPort Utility will not let you create it if they are already in use).
Port forwarding setting in the Apple AirPort Utility for FTP access.
Another port forward setting in AirPort Utility, use the port range set in the ReadyNAS passive port field.
Connection type is FTP with TLS/SSL. I used my public facing static IP as the address and the port that was changed from the default of 21. Root Path and URL are set at the default, did not change it. Passive mode is on.
I hope this helps you out if you’re having issues as I did. If you have any questions just let me know and I can try to give you a hand!