What is the brand and model number of the box you have your coaxial cable plugged in to that connects to the internet?
Are you leasing this equipment or do you own it?
You mention above you are bridging an interface from this box to another box. What is this other box model number?
Wondering is you can look at the diagnostic page for your modem? (download / upload streams data?)
Best support and help with your XFinity connection is via the XFinity forum or DSL Reports XFinity forum and not a phone call.
Here have used Motorola purchased Surfboard Modems for over 20 years now. Currently using an Arris SB6190 for last 5 years.
Arris is owned by Commscope today. Commscope used to be owned by the Carlyle Group. Commscope owns Arris, Ruckus, Avaya.
I worked with ARINC (air to ground traffic) a few years back which was also purchased by the Carlyle group.
Since the beginning here have never leased any equipment from Comcast / XFinity. Currently on a no contract grandfathered account with XFinity.
Same with Direct TV except that AT&T tried to slam my account when they purchased DTV. AT&T has been on my S-List now since the late 1990's.
I have had XFinity issues. My coax line runs directly to modem here as I do not use TV service. XFinity replaced the coax here and it was better.
I went outside with the tech last time to see that my signals were the same at the outside box and at the house. I want to go to Gb but will not until I have a clean signal / line with XFinity.
Tested a new Arris SB8200 (now discontinued) and had issues so returned it.
BTW XFinity shares your wireless on their leased equipment today. That is how they have built out wireless footprint.
Here base infrastructure has always been:
Internet coax ==> Modem (always Surfboard) ==> Firewall (PFSense) (Years ago it was Smoothwall)
==> managed Gb switches
==> Ruckus WAP(s)
Today the PFSense box has multiple interfaces. I use a T-Mobile LTE modem as a failover to the XFinity modem. (2 WAN links) and 4 internal LAN connections. (separate networks or VLANs)
As mentioned above you pay for a speed tier with Comcast / XFinity.
Docis 3.0 modems are different than Docis 3.1 modems relating to speeds and number of download / upload "channels".
When your modem boots up it does a
TFTP to an XFInity server and looks up your MAC address / serial number. It then
downloads the configuration for your modem which includes your speed tier. The modem itself is a router but you cannot see the configuration of it.