MODEM CONNECTIONS

W.E HAVE TP LINK 16X4 ac 1750 CABLE MODEM ROUTER WIFI  680 mbps DOCSIS 3.0  . iT WAS ACTIVATED AND WORKING PROPERLY.  WE RECENTLY ADDED TELEPHONE SERVICE TO OUR XFINITY ACCOUNT. WE PURCHASED ARRIS SURFBOARD TM822 DOCSIS 3.0 VOICE TELEPHONE MODEM. WE WENT ON THE XFINITY WEBSITE AND AND ACTIVATED THIS MODEM AND THE TELEPHONE IS WORKING. NOW WHEN I TRY TO CONNECT TO THE WEBSITE WITH MY TP LINK IT SHOWS THAT I AM CONNECTED TO  THE MODEM BUT WILL NOT LET ME CONNECT TO THE INTERNET EVEN THOUGH IT SHOWS I AM CONNECTED. WHAT DO I NEED TO DO TO CORRECT THIS PROBLEM?