Internet speeds slow even after upgrading service

Hi,

I am in Phoenix AZ. I have upgraded to Internet Preferred 150 plan with speeds 150Mbps/ 20Mbps

But when I run the Internet speed test on COX website only goes at the most at 88Mbps/9Mbps (I tested it via wifi and also connecting my self thru ethernet but it is same results) Noone really using the wifi, only me in my laptop?! How is this possible this being so below the speed I am paying for?

My modem/Router is ARRIS SBG7400AC2 24X8 WIFI MODEM

I am running in 5 GHz speed all my devices in Auto Channel selection.

Since the upgrade, I have reset and restart all my devices including my Router (Manually off/on, and COX did a software restart too).

All devices are connected and I can see the Wiffi network no problem... just the speed is my concern why so slow even after increasing it. please explain why? and how I can get to increase the speed to get the maximum of what I am paying 150Mbps/ 20Mbps?

Thank you!