Is there any chance you can replace your ISDN modem with a "real" network-based one? That would fix all your problems and still allow all your PCs to have access.

Otherwise, if the iPronto is successfully connecting to your network along with your other PCs, you might be able to set it up for net access just like your other PCs are set up - but you'll likely have to enter all the network information (like IP, gateway, DNS) manually in the iPronto.