$50 Remote Service Appointment Fee

https://imgur.com/5UGOfF2

I've had three different technicians out to fix these speeds. The third tech said it was the modem I owned and charged me $50 for the visit. Today I went and got a rental modem from Cox. Still the same speeds. Customer service is now refusing to credit back the $50 even though the issue was clearly not the hardware I own.

At this point I just don't want to be charged for that visit and deal with the degraded speeds. I just don't like being taken advantage of by my ISP. From what I understand, the reason I was charged is because the tech came to the "conclusion" that my modem was the issue. By using a rental modem, I have proven that is not the case.

What can I do for this charge to be removed?