From you description, it sounds like you might be using an xdsl modem card that slots into your machine, and not an external xdsl device.
(if not, disregard below)
The OS has to be able to properly configure these cards, where the external devices save all the settings onboard (managed their onboard web admin pages.) If you can, switch to a stand alone device, it will give you a lot more functionality/flexability. (I think I might have a cayman xdsl modem/firewall in a box someplace if you want to try it.)
Setting the modem up as a bridge (and not a router) will allow it be complete transparent to your network, saving you yet another NAT'ing layer.