$100 extra unexpected bill

I received the xfinite modem on November 11, 2023 and after getting it home, their online apps failed to connect the device with server, then I called the customer care the lady asked me to restart, and troubleshooting, finally she was able to connect the device. However, I took the 1000MB per second package but their device does not had the speed more than 100 MB per second from the installation day. I called again and again multiple times to asked them why the speed is so weird, am I not to get 1000MB per second. The another lady in another day was trying solve the issue and failed, then she said, I am going to send the technician to solve the issues. I asked the lady clearly, are you going to charge me as you are not able to solve your device problem. It was not the problem of my laptop as my laptop was working perfectly with other internet service. The lady said, no we are not going to charge you, we will cover the charge. Even she did not said that we will charge you $100 if it is your laptop issues. If I knew that things I would return back this xfinite modem that day and get another company internet. Anyway the technician came to my apartment, did  line check and told that our customer representative install it in a wrong way. When they install over phone, every time, it has happened. Then he renamed the device and did some troubleshooting of the cable line and then the speed goes 400-500 MB per second.  Still I am not getting the speed that I paid for . Even their internet does not work when I am connecting VPN for my remote work. The speed is weird. Now at the end of month, they send me unexpected $100 service fee for the month of December 2023. Why you are charging me $100, it is your fault, your customer service failed to provide secure and well connection to me. It was your device and technical system fault, not my laptop or phone problem. I am requesting to remove this charge totally from my billing account. I believe that xfinite wants long time customers, not for a couple of months. I am super disappointed about the connection, your low quality service and unexpected bill to customers. You should not give extra pain and headache to people who are using your service. Please improve your technical support personnel,improve your internet speed and do not send unnecessary bill to harass customers.

[Edited: "Personal Information"]