If the signalling server is down, then no new room can be created. If you are missing the main hub, no connexion can happen, it is basic server/client logic here, and nothing much that can be done apart from making sure your signalling server stays up to allow connexion.
And if the host of a room quits the room, the room doesn't work/cannot be accessed anymore. In that case, don't rely on your players to be hosts, and rather make sure your players join room that you host yourself on your server.
The project you provide seems to be working as intended.
This is all indicated in the multiplayer manual article and the tutorials that explain how multiplayer works.