Internet Speed

I have the gig extra plan so I'm supposed to be getting 1200mbps, but I'm only getting 453mbps. I tested with a wired connection because the support articles I found before starting this message said not to test speeds over WiFi. I have the most updated modem and have tried new modem and cables that I got from an xfinity store yesterday, but I'm still having the issue. Can I get some help? I work from home and depend on fast speeds to allow me to do my job as adequately as possible.