Xfinity Modem Perfoming Worse Than AT&T Wi-Fi extender

I got my new Xfinity two gig “high speed“ Internet on Friday. although over the past couple days, I’ve seen everything but that. while trying to figure out what was going on with my latency, I tested the XB8 modem against an AT&T Wi-Fi extender. I plugged the ethernet cord into both and compared the speed, and somehow the Wi-Fi extender was doing better than the whole modem. Keep in mind I have no other devices connected to my modem besides my Xbox and still the XB8 under performed somehow. I’m starting to feel like buying Xfinity was a waste of time and money, I had to have somebody come over and drill holes in my wall so I can get a coax cable installed, which took me out of $300. I have checked all the settings, the gateway page, have tried port, forwarding, changing configurations, and still nothing has changed. I am extremely unhappy with this and don’t understand what to do