Internet Issues - Being held hostage

For a few weeks I had encountered some slow speeds, to be honest I was not worried about it. I am a teacher and was enjoying being away from a computer after so much remote teaching. However, I needed to start working so I called Xfinity. After hours on the phone and promises from tier 2 to call me back, they did not. I spent hours on the phone with them trying to rectify the situation. Long story short they sent me a new modem. That modem took 4 times to be recognized. The average download speed is 2.4. Hard wired it varies from 5-9.2. Open instagram and youtube at the same time kills the connection. As I write this, I am being held hostage by "Mark aka Liza" an advanced technical support person who insisted on telling me about their transgender life. He/she was willing to help until I told them that when I got the modem there was a ton of other devices that were trying to connect to it ( you can see it in the app and online under your account). I told her/him I was concerned that they would send a modem that had not been propely cleaned up.  Now they are demanding screen shots of their shoddy work and here I sit with not being able to open 2 tabs at the same time while I get the notice that my bill is due and autopayment is on. 

I will not even talk about the fact they want to charge me 79.00 to come out to look because since I was sent a modem, it would be considered a hook up fee.  

Sooooooooo I am reaching out here to see if anyone has advice.