Microsoft FTP server configuration

I have a Microsoft FTP server running on Windows 2000 Advance Server. It is running OK for the LAN users but it does not work on internet (WAN) users. FYI, it does not go through a cable modem with address translation. I did not open the port 20 & 21 for the ftp connection. The internet user can connect to the server but they cannot list the directory or upload/download any files. If you do upload the file, the server has a file but with zero byte.

Would someone can tell me what is the problem and let me know the solution? Is it on the FTP server or the cable modem (router)?

The feeling I have is the router since I can user FTP on the local area network which is not going through the router.

Tks & rgds
Edwin Chan