xfinity slow speeds

I just "upgraded" my internet plan through a promotion that was sold to me when I asked for a new remote. This was a huge mistake. My internet speeds are now ridiculously slow. They were much faster before. I had similar problems before. The tech came, said the line outside was bad, also said the modem was bad, replaced both, and suddenly my internet was much faster. I was thrilled. Then I contacted Xfinity to get a new remote. The guy that spoke to me (via online chat) scammed me into "upgrading" my internet so it would be faster by offering about $20 off my bill every month. I've been tricked into these scams before so I guess I should have known better. Now my internet is even slower than it was BEFORE the tech came. Customer service is pointless. They do all of their little tests, and they say everything is fine. (Except they say they are getting 400 while my plan is supposed to be 12000 - and they say that's acceptable. I'm getting about 250.) They "escalated" my issue to some tech team that never called but claimed to have checked my network thoroughly. They said it was fine. I let them know it was far from fine. Now they are sending another tech out in about a week, but this did NOT start happening until I changed plans. Why would a plan that is supposed to offer faster internet actually cause slower speeds? How do I go back to my old plan since this seems to be the issue? Basically, how do I get them to actually fix this??? (Bypassing with VPN does not work, just makes it even slower.) Modem reset does not work. Whatever else they do on their end does not work. I'm stuck, and they are telling me I signed a contract. Well, they are NOT upholding their end of the deal and I should be able to cancel it. I'm about ready to go to another provider for internet.