I'm not sure that I understand your network configuration.
your modem is used as a router and has 192.168.1.1 for internal ip, and wl-500g is the ftp server with lan ip 192.168.1.2 ?
but what is 192.168.0.1 ?
if this is the case, you just need to map port 21 from your modem/router to the wl-500g's internal ip address.
Search for a function named "Port forwarding or Virtual Server" on the administration console of your modem, and map port 21 to ip 192.168.1.2.
Check also if the firewall of your modem doesn't block port 21.
And be sure to connect your router on the lan interface of the wl-500g.