uh-oh...nope, maybe you are configuring a different thing. I mean: it works this way: you have a router, which connects on one side to your ISP (via cable/DSL modem...whatever) and on the other side it connects to your Computers. So when someone attemtps to connect from the outside to your computer running aMule, it gets to your router. But your router has no clue about which computer is running aMule, so you tell it that all "messages" sent to aMule'r port should be sent to the computer using aMule. That's what port forwarding is more or less. Now, your router belongs to you. Why does your ISP messes with it? I don't get it. Also, you should not need a fixed IP. Just configure your router to assign the same IP address to your PCs by using their MAC address (it can be done on my old netgear RP614, I guess all new routers can do that).
Regards.