Years ago when I asked Xfinity about running a server off my Colorado broadband service, they said it was disallowed. I wonder if this has changed or if they’d detect the Freedom box and disallow it.
If I order a freedom box then my coax cable enters my Motorola DOCSIS 3.1 cablemodem which is Ethernet attached to a Linksys Velop mesh WiFi unit. I suppose then I attach freedom box to the Velop node.
I currently have Xfinity and am based in Portland, OR. Port 25 is blocked so I can’t run a standard email server, but otherwise I don’t have any issues running a publically-visible server here regardless of hardware. I’m just speculating here, but perhaps the technician you spoke to on the phone provided a canned answer to discourage the practice of self-hosting on their network.
I am also using Xfinity. They provide public IP address to each user. My FreedomBox is connected to a Wi-Fi router that is connected to DOCSIS cable modem. The public is set on Wi-Fi router and I used port forwarding to route traffic my FreedomBox and Dynamic DNS to get a freedombox.rocks domain and handle dynamic IP address.