BT Infinity is supplied with a VDSL modem (BT openreach box) AND a BT router. You would plug your 2820 (WAN2 socket) into the BT openreach box thus replacing the BT router.

---(fibre)---> Cabinet/green box ---(copper)---> BT master skt ----(rj11)---> BT VDSL Modem ---(cat5)---> Router (BT or other)