issues with internet after modem change

Hi, Xfinity offered to ship us an advanced modem(modem+router combined in one), i think this is the XB6 kind of modem.

Evere since we changed our modem our internet coverage has become quite unreliable and choppy, it goes down quite often, the xfinity site suggests that we reboot the router after a few minutes, sometimes they ask to loosen the wire that is connected to the modem, it has not been a good experience ever since we installed the new router cum modem. can someone from the xfinity team look into this issue or help us in someway please.