Why Can't I Get The Speed I'm Paying For?

I have an Internet only plan with Xfinity. I originally had 300 mbps plan but over the past 2 years was upgraded to 600 mbps (paid for this) then updated to 800 mbps and then 900 mbps for free. When I was updated to 600 mbps and ran SpeedTest I would get 600+ mbps all the time at the Ethernet ports. In April 2020 I decided to get my own NetGear Cable Modem and Router to save money and have a stronger WiFi signal. I got a NetGear 1150 which was advertised as DOCSIS 3.1 up to 800 mbps at that time. I got it setup and activated. However, ever since being upgraded for free to 800 mbps and then 900 mbps I cannot reach these speeds. I’ve had numerous calls to Xfinity Advanced Technical Support. They would send a command to the modem and it would jump up to the speed I’m promised. However, within 30 minutes, it would drop back down again to 600+ mbps at the Ethernet ports. I had a Xfinity technician (Kyle, he was excellent!) come to my home last week to check out my lines. He replaced all of the cable connectors in the house and gave me a new cable between the wall outlet and the cable modem. I had prior to his visit bought new CAT6 cables to go from my modem to the router and from the router to my iBuyPower Gaming PC. He told me that before he came to my home, that he noticed that my Cable Modem had reset itself 52 times overnight! He told me if I logged into the Modem and looked at the Event Log and it would show this. Before he left, his device showed the signal at 900+ mbps. I tried to login to my modem but it wouldn’t take my changed password or the default. I disconnected it from everything and turned the power back on and pushed in the Reset button for 10 seconds to completely reset it. I then connected it to the cable line and after about 15 minutes it was working properly. I connected it to my Router, turned it back on, logged into my PC and ran Speedtest. I was finally at 900+ mbps. I checked 15 minutes later, and it had dropped back down to 600+ mbps. I checked the Modem event log for several days and it was empty, no new events. I have found that if I go to the Xfinity Support Site and click on Restart Modem, wait for it to complete, turn off the Modem, unplug the power cord for 30 seconds, plug it back in, restart the modem and when ready restart my TP-Link Archer AX6000 router and run a Speedtest that I’m at 900+ mbps again. However, when I check 15 minutes later, its dropped right back down to 600+ mbps. So, I decided to get a new NetGear CM1100 Modem at Costco, rated up to 2GB mbps. I came home, disconnected the old modem, plugged in the new one, activated it with Xfinity and connected it to my TP-Link Archer AX6000 router and started it. I ran a Speedtest that I’m at 900+ mbps again. However, when I check 15 minutes later, it’s dropped right back down to 600+ mbps. I’ve come to the conclusion that while Xfinity says that I have a 900 mbps plan, that I’m being throttled back to 600+ mbps by them. I have no other way to explain why I cannot get 900+ mbps consistently. Has anyone else had this experience or found a solution? I’m at my wits end. Thanks in advance for any and all replies.