How to get help for dropped packets, MDD message timeouts & uncorrectable codewords

I have had internet connectivity problems for over a month now.  I'm not understanding how to get help for it.  Twice when I've gone into the system, it's immediately taken me to a page to schedule a tech.  When they come, they say the signal is "fine".  I have weeks of data collected from my modem that shows it is not fine.  When I ask, how I get further help, they don't respond.  I rented a modem from Xfinity hoping it would allow them to troubleshoot, but the self-diagnosis again tells me everything is fine.  This is not "fine" to me.  How do I get help for this.