In case anyone else encounters this issue:
I installed software version 4.1.2.56 (Win XP Prof) for the 3CRPAG175 XJACK wireless PC Card. The USR9106 ADSL wireless gateway has version 2.5.1 firmware installed. If in USR9106's Wireless AP>Advanced Settings | 54g Mode is set to "54g Performance" there is no connection, however if 54g Mode is set to "54g Auto", connection is successful. ["54g Performance": "...select the mode to 54g Performance for the fastest performance among 54g certified equipment..."]
With the older 3CRPAG175 software 1.0.0.36, connection was successful with the mode set to "54g Performance", so the above issue was a surprise.
Lapimate