Please read the link I posted. Figure out whether you are using Passive or Active FTP on the client. If you are using Active you will likely have to forward the Data port/connection on the client side. If you are using Passive you likely will need to forward the Data port/connection on the Linksys(server side). Port 21 is the control port/connection.
As I said most routers have special routines that deal with FTP because it uses 2 ports and therefore when you are not using port 21 they may not know they need to allow the connection on a different port.
If you're not using one of the ports listed in the FAQ, it would not be blocked by the modem. It would either be an issue with the FTP software or router configuration.
