Well there are no guides supplied for your modem at Port Forward.com, so I googled it and couldn't really find anything about it having a Nat Firewall, as it's just a modem not a modem/router...
There is nothing else in your details that indicate this firewall either...
So I am going to suggest trying these fixes as they have worked for others....
http://www.gnutellaforums.com/connec...t-fix-try.html