Slower speeds after Xfinity modem/router upgrade

My last Xfinity modem started causing error messages in my antivirus saying that Xfinity's certificates were expired. When I talked to an Xfinity representative regarding this, they said I needed a new modem. I thought this was strange, but I complied. I was charged for the replacement modem as a new setup, was sent a new modem and a flex device (which isn't part of my plan), and am now getting 900mbps/1mbps upload vs the 1350mbps download/120mbps upload+ I used to get, and that I pay for, even when I'm the only one using the internet. I'm connected via Ethernet, using the same cable as before. The only thing that has changed is the modem, and the coax cable used (I'm using the new one). Using the Xfinity app, I can see the modem is receiving around 1400mbps speeds from Xfinity, but my devices aren't receiving it.