It has been my experience that when you are behind any router, you can't DCC send. One thing you may want to try is DCC send from the server, not the client machine(s). If it still does not work, you are probably out of luck.
Pentium 3 1000 @ 1100 ( windows 2000 server), Abit bx-133, 256 Mb Mushkin Rev.3 cas 2-2-2, ATI Radeon
AMD Thunderbird 1.4 @ 1.63 (24/7) IWILL KK266R , Corsairs pc 150 cl2, Leadtek GeForce 3