After activating new service, NOT getting advertised speed

Hi,

I just recently got new Xfinity Internet service (PERFORMANCE PRO+) and the advertised speed is 200Mbps.  After I activated the service, I got the speed upto 200Mbps for the first 4 days.  On the 4th day, my speed dropped to 150Mbps and it is very consistent the whole day, and I tested it  by direclty connecting my laptop to the Modem using Wired ethernet cable.

I closed my existing account (150 Mbps) and got new service (200 Mbps) at the same address (With 12 months contract) in order to get the lower monthly price. 

I suspect, since I moved the Modem from my previous account (which is 150Mbps) to the new account (200Mbps).  Tried to call Xfinity support over the phone, but not helping out, as they just simply restart the modem and say the boot profile if correct.

Can anyone in the forum help me out?  Thanks.