You have a crappy router, ditch it and get something good like a linksys, one that doesnt try and alter the dcc send messages on the fly. I bet its a dlink, burn dlink burn!

PS: your router is being way to clever for itself, and is altering what port you use for dcc on the WAN side of the router becuase it thinks it should and it knows best, I doubt you can fix its smarmy beleive that it knows best, besides with several blows of a hammer, which is likely to do other unrecoverable damage to it, while your teaching it whoes boss.

PSS : was it a dlink?