Let's Encrypt can't work on Cox
I just want to restate that Cox blocking port 80 started out as an "okay" idea for security but has become the exact opposite. All Cox-provided routers have a firewall, and the majority of people not using Cox hardware have their own firewall because that is their preference.
In my case I want to use Let's Encrypt to provide certificates for my firewall, and Cox makes this impossible by blocking port 80. Please read up on the ACME spec and see if you can do something for it.