Troubleshooting FTP server creation in Snow Leopard

I've got an old Mac Pro running 10.6.8 that I'm trying to turn into an FTP server that can be accessed remotely. (All I want to do is use this computer as a box to which a few colleagues can send files.) My biggest hurdles are that I know nothing about networking and I don't have any money to spend. After looking up a bunch of tutorials (especially this one), here's what I've done so far:

-Enabled sharing via FTP & AFP on this computer

-Turned on port forwarding in my router (port 21, TCP, on the computer I want to be the server) via my router's portal

-Registered a second-level DNS pointing at this computer's IP address at no-ip.com

-Downloaded an FTP client on a second computer for testing

And here's what I've accomplished:

-File sharing on my LAN is going just fine

-I can't access my would-be FTP server through the FTP client

Can anybody help me out? The guide I linked above recommends using DynDNS, which was free at the time of writing and is no longer. (That's why I used No-IP instead, but I don't know nearly enough about this subject to get it to work on my own.)