How to fix you seem to be using a VPN or proxy, when you don’t have a VPN or Proxy?

It would be interesting to see if the cable change fixes it.  I'm guessing it won't, but perhaps there is something about your modem that I don't know.  More likely I think your public IP has gotten on some VPN detection service's list of bad IPs and that VPN detection service is used by whatever streaming service is giving you that message.  Sometimes this happens legitimately because the user has an app on some device in their home that functions as a VPN exit point.  (This is often noted somewhere around page 12 in the Terms and Conditions that they didn't read.)