Slow and Inconsistent Speeds on Gig plans.

I've recently upgrade my internet speed from 200mbps to the gig speed, preivously I had no issues with getting 200mbps and well above sometimes into the 240s on hardline I would consistent speeds. After my upgrade I've been struggling to recieve speeds above 450 on wifi, but isn't the only issues the speed would be inconsistent as well one speed test ill get 450 another one will jump to 300 and then continue to drop back down to 80, I've had speeds as low as 10mbps which is insane for the price I'm paying for. I upgraded my modem from the cm500 to the cm1100 and verified with the tech over the phone that this was a modem that was capable of getting gig speeds. I've had the plan for roughly about a month and I've called comcast about 4-6 times each time getting the prompt that my modem needs to be reset and I've troubleshooted with 3 different techs with no succes. I was recently told that my modem was broken and that comcast could do nothing for me and to buy a new modem, I was also put on a waitlist for a call back from comcast for a tech to come over and look at the wiring, but never received that call back. The tier 2 tech support guaranteed I would get a call back within 24 hours it has now been over a week. If at&t fiber was available in my area I would switch in a heartbeat.