ive gotten 2 new modems that on the box say are capable of 800mbps down however when i run a speed test with my PC and it being the only thing on the network, im barely getting 200 down. i pay for 800mbps so im trying to get at least 500-600 down. contacted XFinity and they say its a modem issue but i cant get ahold of an agent via live chat because the site cant find its own modem. bought it at best buy and i just have no clue what to do. any tips?