You'll have to get into your modem configuration and copy the VPI/VCI numbers of your existing configuration, then create a bridge (not an internet connection, what you are using is an internet connection) with the VPI/VCI numbers of the existing connection. That way you'll be able to dial-up with your computer.
I have done this on my own modem so as to attach another dd-wrt router dialing up the connection and running chilispot to it-AP mode and DHCP sharing does not suit my needs.