Motorola sb5100 + 3com 3crwe554g72 dhcp problem

Hello.

I have an annoying problem with my 3com router since i got a new cable
modem. When i plug in one of my computers directly to the motorola
cable modem the computer obtains a dhcp address and surfing works just
fine.

However, when i plug in my 3com router between the cable modem and my
computer i receive a local ip to my private lan but the router is
unable to obtain a public address, and therefore, surfing the web is
out of the question.

I wonder if anybody has experienced the same problem (even if it is
with another hardware setting) as i have and if there is a way to come
around this problem?

Kind regards

Reine A