Is it simply a modem? It may have in-built NAT or/& other kind of protection. Westell are known for issues. I presume it attempts to use UPnP right? You might be best port forwarding the Westell device for ShareAza.
A quick search, seems your model does have NAT, etc. portforward.com forum - Westell C90-610030-60

Unless I'm mistaken, this is your model: PortForward.com - Port Forwarding Shareaza on the Westell C90-610030-06, if not it, the instructions will be very close if not the same as yours. I never was good at maths, who cares about carrying a one or zero across. lol

Always first step is to set up a Static ip address otherwise the port forward process will not work.

OldDeath will be able to help you with ports needed for the different networks ShareAza connects through. Default 'official' port for Gnutella network is 6346, however using this specific port is not necessary, basically any port can be used so long as it does not conflict with other officially used port numbers.