I often find it challenging to find specific Xfinity web pages used manage my Internet service.
Here is the list of URLs that I am often struggling to find.
Status, and Remotely restarting the modem
https://www.xfinity.com/support/status
Under "Internet", there should be a Cable Modem entry that says "Connected" and which has a Restart button.
Monthly Data Usage
https://customer.xfinity.com/#/devices#usage
Also in JSON form at
https://customer.xfinity.com/apis/csp/account/me/services/internet/usage?filter=internet
Hotspot List
https://customer.xfinity.com/#/settings/security/hotspot-devices
Speed Testing
This is my go-to speed testing service. It provides a lot of great information. It isn't a Comcast service, but it does a great job.