500/500 Mbps Symetrical Intenet Plan - Upload Speeds Capped at 100 Mbps

Hello - I just upgraded to the 500 Mbps download/500 Mbps upload plan and my upload speeds are capped at 100 Mbps, download speeds are in line with expectations. All of my speeds were measured using a wired connection to the modem/gateway.  I previously used my own modem (Arris SURFboard Model SB8200v3), but recently found out that that modem will only support up to 35Mpbs upload through Xfinity.  I recently upgraded to the Xfinity gateway (Model XB8) and my upload speed increased from 35Mbps to 100Mbps.  While that's better, I would like to utilize all of my upload speed.  Is 100Mbps all that I can expect from this particular gateway? Or, is there something upstream of my home that needs attention?  I'd like to arm myself with some information before navigating the customer support phone tree.  I had attempted to resolve this through Xfinity Assistant chat, however it was not able to resolve my issue.  Thanks in advance.