Im having the same type of problem with my FTP server that i could only reach within the Lan and not the Wan.
Here below is my current set up, hopefully someone could give a hand to sort it out and make it work from the internet.
1.>My 2 computers connected (wireless) to a linksys (Wrt54GL with factory settings, Firmware 4.30.7) which is also connected to a Modem
2.>The automatic DHCP on the router is enabled and Ip range is 192.168.1.100/120
3.>The computer with the FTP server (server-u V 6.0.4) has got fixed ip address 192.168.1.12 and could flawlessly connect to the internet
4.>From the linksys GUI page>>Applications & Gaming>>Port Range Forward i set an entry for the ftp server and forwarded port 21/Tcp to the machine hosting the server (192.168.1.12)
5.>I registered at DynDns and get a xxx.dns.org domain address
6.>In the linksys GUI page>>Setup>>DDNS, i enabled DynDns.org service and provided my xxx.dns.org address in the Host Name field
7.>In the Server-U administration page>>Domains, i put the name and ip address (192.168.1.12) of the computer hosting the server
With the above settings, i could type in any of my browsers
<a target=_blank class=ftalternatingbarlinklarge href="ftp://xxx.dns.org">ftp://xxx.dns.org</a> and get to my server. However when people from distance try that same address, they get a time out message from their browser.
NB: a) my Modem is a Modem/Router and both Router & DHCP features are still enabled
b) in the linksys GUI page>>Status>>Internet, the ip address stated is 192.168.0.3 which is derived from the Modem DHCP automatic ip assignment.
Thank you for your reaction & help.
Edit Point: Good Lord i forgot to mention that i have ZA firewall running on and i granted full access to the Ftp Server.