@VW618 -- if you move from your "man-cave" to another room (toilet? beer fridge?) in your home, you probably do not need to go outside your home. Similarly, connecting from one computer connected to your cable-modem to another computer connected to the same cable-modem does not require that your cable-modem have an active connection to the Internet.
If you go from inside your home to the outside, you will need the key to your front-door to re-enter your home. Similarly, when connecting from a computer on the Internet to a computer inside your home network requires that your cable-modem "allow" the traffic to connect.
So, try to check the configuration of your cable-modem's "access-list", to ensure that such unsolicited, external, traffic is allowed.