Charged $100 for Professional Installation

I switched to XFinity Gateway and Giga service to alleviate ongoing network drops.  Picked up the kit and installed the new gateway, but the problems persisted as they had nothing to do with the equipment in the house.  The service tech determined a major issue in the outlet box outside of the house and other infrastructure issues that made the upstream power go out of spec.  They put on a temporary fix, but the entire run from the box to my house will need to be replaced as it is about 30 years old.  

Why am I being charged $100 for installation of new equipment where the issue was and still is the aging Xfinity equipment outside of the house to fix the network.

Please Advise.

Al