Upload speed upgrade with Arris SurfBoard S33

Hi,

My Xfinity account says my service is eligible for 100Mbps upload speed.  I am having my own Arris SurfBoard S33 cable modem, which is also listed as a compatible device on Xfinity page.  But I am only getting 20Mbps upload speed.  Rebooting the modem didn't help, either.  How can I enable the faster upload speed with my own compatible S33 modem?  Thank you for the help.

-Dennis