I'm not sure about your particular model, but with a Nokia M1122 I just navigate in a web browser to:
so you might be able to do something like that for your model. Otherwise try:
and you should be able to connect to the router's admin interface like that.
Then you'll need to look up a section entitled either NAT or NAPT, all the settings you need to change are in there.
I've done this once before and it worked OK, however the M1122 is an all-in-one hub/router/DSL modem.