Unless you're using DCC server (passive dcc), connection problems are the *sender's* fault. If you're receiving these files as you say, your router has nothing to do with the problem-- the users who you are receiving from need to configure their routers to send.
Alternately, you might want to configure your DCC server as mentioned above-- this requires also forwarding port 59 and enabling DCC Server in Alt-O -> DCC -> Server. This might fix receiving for those users.
