Some home network connections are being forwarded to an external captive portal

Some connections to my home network are forwarding to an external captive portal site - connectivitycheck - which brings up an xfinity mobile hotspot login page, but this is not a mobile hotspot - it's my home network. How do I stop this from happening? Not every connection gets forwarded to the external site - only some of them. This includes some of the wifi connections and some of the ethernet connections - but not all. I know they are connected to the internet because they are bringing up that external page, but they show as not connected because that captive portal appears to be blocking the connection. But again, some wifi connections and some ethernet connections are working fine.