Poor download speed

We received Xfinity service yesterday. We are using an arris sb8200. When the tech left we were getting 200mbps download speed out of our 600mbps plan. My room wasn’t happy with the speed when I was updating my Xbox because it dropped down. So he started messing with the modem, and eventually couldn’t get any internet. I reset the modem and got the internet working again, but now we can’t get over 50mbps download speed. Any idea how to fix this situation and get it back higher