Got me stumped. Take a look at the following link & check your model under the UPnP section. There's a link there that discusses that model's firmware issue: Connection problems? Check here first! (click on link). But only take notice if you're brave. It's worked well for those who have done it, but it has major risks. So I reiterate, only take notice if you think you know how to deal with it.

You might also be interested in this http://www.lvllord.de/?url=tools

Actually a good port range to connect through is 49152 - 65535. These are informal ports not used for anything officially. A good one is 64049

You could also try Phex to see if you have the same problems.