Arris Sb8200 slow speed with xifinity

So From last 4 years i am

having same speed issues with xifinity with my own devices( arris sb8200 modem & AMPLIFI mesh wifi router). I have change multiple plans from 600mbps to 800mbps now 900mbps. So recently i change my plan to 900mbps but unfortunately i never get the speed Xfinity promise me. I had almost 6 visits in past from

technician where one told me that i have old modem so i change my modem to arris sb8200 from previous lower arris modem. But still i am only getting 250mbps barely on ethernet with sb8200 directly linked to LAN port and 220mbps on wifi with AMPLIFI mesh router. Nobody yet resolve my issues. Support people using ping pong method to resolve issue in past. Today technician came in and showed me that bandwidth is good with upstream & downstream but even he didn’t figured out why i am still having this slow speed issue even sb8200 is Xfinity compatible. Please if anyone knows anything which can help me