Internet speed

My Internet speed is typically about 120Mbps, measured on an Ethernet-connected computer using xfinity’s online speed checker.  I contacted xfinity via live chat to see if I could increase it.  The representative said my plan provides for 300 Mbps.  He tried for nearly an hour to reset the modem, etc., and nothing changed.  He scheduled a service call because he said the problem must be in the wiring.

I looked online and my plan calls for “Extreme 105 Internet Speeds up to 105Mbps.”  Before I have a service call, I’d like to determine if the representative was mistaken, or if what I see online is wrong, and if there’s anything that can be done without a service call to increase my speed to 300Mbps.