Being charged installation fee because it's within 30 days of starting service, even if it's not an installation issue

Hello, I've been getting 100-200 Mbps speeds since installing my modem/router despite paying for the 1000 Mbps plan.  I contacted customer support who scheduled a technician visit, and I was charged a $100 professional installation fee.  I was told by the first customer service agent that their testing was concerning for an external wiring issue.  I discussed this charge with a second agent who confirmed that this $100 professional installation fee could not be waived.  However, as I installed the modem/router correctly, and I have done so many times in the past, and this appears to be an external issue, I would like confirmation that if it is an external issue, that my fee will be waived.  Otherwise, I am essentially being charged $100 to receive the service that I am supposed to be getting without this extra charge.