Mentor probably is some cheap shit - most comments on the Net are negative. According to a descriptive ad-like site I found it at least is supposed to handle port forwarding.
at:
http://www.linden-computech.com/pc/adsl_mr4.htm
Back to basics: Do you know your IP-numbers (two of them, internal and external)? Seems so as you can connect to the router (?) but not to the machine.
Ok - you do. Then port forwarding is the thing.
You run your server at some port, maybe 21, at your computer. So, the router needs to know that all connections from the outside asking for port 21 are to be forwarded to the computer runing the ftp-server.
Look for port forwarding (or maybe virtual server) in the settings and op redirct the portnumbers you need.
And welcome here to BetaONE!