MB8611 up to 200Mbps only in my 800 Mbps subscription

Despite subscribing to an 800Mbps plan, I'm only getting sporadic connectivity with speeds reaching up to 200Mbps.

An Xfinity technician has just visited and thoroughly checked my setup, confirming that everything seems fine. The only remaining solution they suggested is to replace the underground cable, even though it was replaced just 13 months ago when I upgraded to the 800Mbps plan.

To my surprise, when I consulted Xfinity's list of supported devices, I discovered that the MB8611 modem I'm currently using is no longer supported by Xfinity. I found this information in the following document: https://assets.xfinity.com/assets/dotcom/projects_2/my_device/Full-List-of-Compatible-Devices.pdf

Since May 2022, when I started the 800Mbps internet service, I was never informed that the subpar performance could be due to the Xfinity-certified MB8611 modem, which even has the Xfinity branding on its package. MB8611 firmware updates are controlled by xfinity and my mb8611 shows 8611-19.2.18, the original firmware installed by Motorola.  It's quite frustrating and feels like a joke.

I kindly request Xfinity's guidance and assistance in resolving this issue.