As stated above, you will need a DOCSIS 3.1 device to achieve those speeds. The G36 should support those speeds if you want to stick with a modem/router combo, if you want to get just a modem, I think the S33 will also achieve those speeds. 

One other call out with those speeds, make sure that your speeds are not being bottlenecked by anything else. For example, if you use an Ethernet connection to any of your devices, you will need at least a Cat 5E cable. Your WiFi speeds can also be bottlenecked by the device that’s connecting to it, the amount of interference, both from other WiFi networks as well as environmental interferences. Just some other things to look at when you start subscribing to faster speed tiers.  

Here are the links to the service provider websites where it would list the compatible products to the different service tiers.  Only with the compatible products would you be able to achieve the speed that you subscribe to and pay for.  As others pointed out, other things to look for is WiFi speed, Ethernet connection between your modem and router, etc..  But making sure you have the right modem should be the first thing to check.

https://mydeviceinfo.xfinity.com/

https://www.cox.com/residential/support/cox-certified-cable-modems.html

https://www.spectrum.net/support/internet/compliant-modems-charter-network/?domain-redirect=true

The explanation is about the technologies, your SBG10 is not an DOCSIS 3.1 device and either is an certificate device from Xfinity to work under that newest speed plan, your SBG10 will only work on a speed plans not greater that 300mbs,

In case you will like to upgrade your device for one that is compatible, you may look to upgrade to the G36 that is one of our newest Gateways that is compatible with the 1.2gb connection