No change is internet speed after upgrade. Arris SB8200 and Arris S33

My xfinity plan was 300mbps. A competitor was offering fiber to the door internet for $70/month (the competitor has some issues with reliability, but they are alright). I called xfinity to cancel my plan to switch over to the competition. 
They offered me the 1.2gbps plan for $70/month on a 12 month contract. I said.. ehh xfinity is more reliable and I will be paying the same price, so what the heck! Sign me up. The next day I ran a speed test and I was still at 300mbps.

I called xfinity support and they said I was receiving the full 1.2gbps speed and my Arris SB8200 modem config was the issue

I called xfinity support again and they said that my speed was not compatible with the SB8200 modem. I would need to upgrade my modem to use this plan. (if its not compatible why do I get 300mbps(my previous plans speed) out of the 1.2gbps? )

I called xfinity support again. They said I was getting the full 1.2gbps speed. My modem is the issue and I needed to contact Arris support to resolve my configuration issue. 

I called xfinity support again. They said they passed the issue to T2 support and things should be resolved in 2 hours. Nothing was resolved.

I called xfinity support again. They again claimed I was getting the full 1.2gbps. My modem is not accepting the config for the 1.2gbps connection and to call arris support

I called Arris support and they said I needed to pay them money to talk over the phone since my modem was out of the 2 year warranty.

I went to Arris online chat help. They claimed they cannot change my modem config and recommended upgrading my modem to handle the higher speed, because my modem won't work with the 1.2gbps plan.

I explained that it does work with the plan, because I was currently using the plan with my modem to chat with them on my 300mbps connection. I explained that the xfinity support said my modems config was not correct and I just needed arris support to fix things. They claimed it was impossible for them to make changes to my modem and recommended either upgrading my modem or downgrading my speed

I called xfinity to change my plan to a speed that the xfinity website says my modem supports, but they wouldn't offer me the discount rate

I called xfinity plan cancellation support and explained I would like to keep the discounted rate, but change the contract speed from 1.2gbps to 900mbps. They said okay

I ran a speed test and still got 300mbps

I called xfinity support and they said I was getting the full 900mbps speed. They said my modem wasn't accepting the new configuration required to get the full 900mbps. They said the only way to resolve the issue is to contact arris support.

I went to BestBuy and purchased a brand new Arris S33 modem that handles up to 3.5gbps to the modem and 2.5gbps ethernet port. I activated the S33 on my account, ran a speed test, and I still get 300mbps

It's currently June 14, 2022. I will be calling xfinity support again tomorrow to let them know that my brand new modem is still only getting 300mbps.

What is wrong with my connection speed? Why is it not changing from 300mbps to a higher speed?