The MultiplexTransport is not a transport itself, but it allows you to combine other transports so that your clients can connect to your servers via either one of them.
A common use case for the MultiplexTransport is a server listening to both websockets and TCP. Your webgl clients can connect to the server using websockets and your mobile or desktop clients can connect to the same server via TCP. In HLAPI, you had to chose between websockets and UDP, but you cannot use both at the same time. You can configure any number of transports in the MultiplexTransport.
To use the MultiplexTransport follow these steps:
Add a gameobject with a NetworkManager to your scene if you have not done so
By default, Unity will add TelepathyTransport to your NetworkManager game object
Add a MultiplexTransport component to the gameobject
Assign the MultiplexTransport component in your NetworkManager's transport
Add a WebsocketTransport component to the gameobject
Add the TelepathyTransport component to the MultiplexTransport as the first transport
Add the WebsocketTransport component to the MultiplexTransport as the second transport
Please note that Telepathy and WebsocketTransport cannot listen to the same port. By default Telepathy listens to 7777 and the websocket transport listens to 7778.
If you build your game as a webgl game, the TelepathyTransport will be skipped and your client will use the websocket transport. If you build your game as a mobile or desktop app, it will choose TelepathyTransport. The server will happily accept connections from both.
Here is what the configuration should look like: