What do you mean by a stable connection? No connection problems? Without details about the initial situation (or existing problems) and the desired result, you are somewhat in the dark.

The card works perfectly with all conceivable routers, including the Connect Box. I see no reason why a second router should be operated in bridge mode. That only brings a loss of performance.
If you want to use a higher-performance router in the UPC network, you switch the UPC box to modem mode anyway and connect the second router to the UPC box via cable.