Of what I understand of what you are trying to do, bridging is the way to go. That way you 'shut down' the router part of the modem and just use the modem capabilities.
You should be able to set brigde networking using to config software you received with your modem. If not see the vendors website or contact your isp.
I use a speedtouch 510 in the Netherlands and used this link to setup bridging:
There are some issues regarding settings if the modem regarding different isp in the Netherlands. So if you're not Dutch and not using BBnet/Tiscali network you should look for a config util on the international support pages: www.speedtouch.com
or contact your isp