Completely disconnected from the Internet means that packets can’t make it from the device to the Internet. You achieve this through physically separate networking, VLANs or simply relevant firewalls. With a stateful firewall you can get picky with restrictions on TCP SYN packets to get fine grained control of who can establish a connection. I felt this level of detail was inappropriate for OP’s self proclaimed technical proficiency.

Possibly OP means Internet connected, but not using a third party service to handle the feed. If they do, then they can ignore my comment and select something more appropriate from other helpful contributions.