how to configure adsl router

Clarifications

You will either have, in your router, the ability to forward ports. Depending on router this can have different names.

Or, the ability to set up "virtual servers" which is a similar idea to port forwarding.

Both these methods generally require the local IP to forward too, the port/port range to forward, in your case 20-21, and the protocol, TCP or UDP, your case would be TCP.

Make sure you set up strong strong passwords, don't even have an account called Administrator or Admin, use a different name, FTP servers are constantly and randomly hammered on. If your doing anything private, make sure your FTP server uses encryption of some sort, not plain text.