If your router is sane at all you can use "port triggering" to get this working, simply make a trigger that when connecting on 6666-7000 you open 1024-1029 to your local client. Then restrict DCC to that range.
Alternatively if by router you mean a linux box, you just need the ip_conntrack_irc and ip_nat_irc modules loaded with specified ports(eg: modprobe ip_conntrack_irc ports=6667,1024,1025,1026)
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