New modem not registering AND slower connection

[death2lois]

I just upgraded from 265mbps to 600 mbps a couple days ago.  It took effect yesterday (allegedly).  I ran speed tests before and after and there was virtually no change.  I updated my modem today and this new one is rated at 686mbps vs the 300mbps I had before.  

My connection speed is now 37.6mbps.  

AFTER UPGRADING!!!!

When I try to troubleshoot and reset my modem, It shows the serial number of my old modem.  The setup I went through never prompted me to add information about the new modem.  It just asked to verify my phone number and then sent me a text with a code to verify it was me.  

There used to be a portal in which I could enter the information for my new modem, but there is nothing like that (at least that I can find) on Xfinity's site.  Chat is absolutely worthless (when it actually connects). 

How can I get Xfinity to recognize my new modem?  Why am I paying for more speed, but getting even less?

[CCAndrew]

Is your modem rated by the manufacturer or Comcast for your speed? There’s a difference
What’s the model