charged for service call when it was supposed to be no charge

in october we changed out bundling (dropped voip and returned extra mini boxes) and realized were were not getting the upload speed we paid for.  CSR claimed it was due to our older docsis 3.0 modem.  we agreed that if i bought a newer docsis 3.1 and still not get the upload speed they'd send out a service tech NO CHARGE.  I did (upgraded netgear cm600 to netgear cm1100) and the upload speed was still locked at 25Mbps (i have superfast which supports up to 100Mbps).  a tech came out.  everything is fine with the cabling and the equipment.  the issue is xfinity and netgear (and other manufacturers) can't agree on a protocol for bonding the upstream channels to enable higher speed, or at least that's what the tech said.  still the service charge was promised as NO CHARGE.

today i pay my bill and there's the $100 for the service call.  it was supposed to be NO CHARGE.  it has to be in the account notes somewhere.  can anyone help?

thanks,

jeff