Don't forget that in order to connect your Xbox's FTP server, ports 20 AND 21 will need to be forwarded.
You can have any port you like ie [ip]:30333 (which would mean this port is forwarded instead of 21), but you would still need port 20 forwarded also.
For a modem, it could be easier. For instance, if it's a static IP account you have, just set the Xbox to DHCP and boot with the modem connected directly to it. You should be able to reach it via the internet using the static IP.
Martin
Martin