Poor internet performance... And I'm sick of fighting for what I'm paying for.

This is a long story, but I'll start with the fact that I am on my umpteenth call with customer service at the moment, with an issue that has yet to be resolved after several calls over the past 3 years. We are paying for 600mbps, and have never gotten more than 300mbps. We've gone through the tech support song and dance (reset your modem, wait 10 seconds, blah blah blah) every. single. time. and the end result is pretty much the same every time - the modem we're using is not good enough to support the internet speed we are paying for. Even though it's the EXACT modem they told us to buy on our last call 6 months ago. And all information on the box says it does support that speed and is supported by xfinity.

Last week, I called again, went through tech support, did all the things, and in the end he told me it was an issue we had to take up with the manufacturer of our modem. NOPE. I told him to connect me to a supervisor, which he couldn't, but said one would call me shortly. This didn't happen. So now I went through the same thing, told tech support (about 10 times) I will not be doing any tech support, I just need to talk with a supervisor. He was resistant, but finally put me on hold to speak with someone. I've been on hold for 25 minutes now. Anyone else experience any of these issues, from resistance customer service representatives, so just overall horrible customer service? I'm ready to report them to the Better Business Bureau.