`demo.zenoh.io` is not a public server which is always running. It's online only from time to time when ZettaScale is running demos.

You should try running you own Zenoh router on some host in your LAN, or in a cloud VM instance with a public IP.

See the instructions to install and run a Zenoh router here:
https://zenoh.io/docs/getting-started/installation/#installing-the-zenoh-router