I have the same router and the same problem with AmIRC here. What I did on my PC with Windows xchat was to set 4500 as the only one port for DCC sending, then I accessed router's configuration and opened TCP and UDP ports numbered 4500. It worked flawessly, however I am unable to find such an option on AmIRC (in order to limit the range) nor I don't know the exact range that AmIRC uses. Hope it helps. See you.
