So you are 100% sure, that there isn't more than one DHCP server running and that, smoothwall is not set to distribute ip addresses, if that is the case you might want to turn DHCP off on Smoothwall and allow you modem to distribute the ip addresses or vice and versa.
Another thing to make sure, is that smoothwall has all the correct ip settings, like default gateway, subnet mask and dns ip address.
In my set up, I manually set all the ip addresses and configured my modem to lease the ip addresses, instead of Smoothwall.
You might want to re-check all the configuration in smoothwall. Do also a ping test, to determine if smoothwall can ping any internet machine or external host, like google.com.au