Ethernet adapter cannot connect to cable modem

Here is my situation:
I signed up for ATT's VoIP phone service. I am currenting using Road
Runner cable Internet service which connects to my Win 98 computer with
a USB cable. The telephone adapter requires a ethernet card which I can
install and Windows will detect.
There is a ethernet connection on the cable modem. When I connect the
ethernet cable between the modem and the ethernet card, I lose my
Internet
connection. I receive error various error messages saying that there is
no communication between the two.
I have been on the phone for hours withtech support to no avail.
Any help would be appreciated.

Thank you,