I am trying to develop a small websocket API (as a testbed first) but without using the Phoenix frontend (that socket.js). I need it to communicate to any ws client, not related to Elixir/Phoenix, just exchanging json messages.
Of course I started from the standard, ubiquitous tutorial found online, that one with a simple chat, by modifying it:

• setup Phoenix project – standard install/config, then:
  mix phx.gen.channel room
• started & tested the Phoenix server installation in the browser – running fine

• modified endpoint file:

socket "/socket", TestWeb.UserSocket,
  websocket: true,
  longpoll : false

• user_socket.ex file added in:
  channel("room:*", TestWeb.RoomChannel)
• All others left default/unmodified

As you see, nothing special…

Further on, as I understood that the channel must work out-of-the-box reply by default to join, I tried to connect with wscat: wscat -c ws://localhost:4000/socket
Result:

error: Unexpected server response: 404

Hmm… something wrong with the url then tried also with: ws://localhost:4000/socket/room and ws://localhost:4000/room… and several other combinations. But got the same result : 404
Maybe wscat is faulty… another series of trial connections using another websocket client, but with similar results.
On the server side, Elixir reports:

[debug] ** (Phoenix.Router.NoRouteError) no route found for GET /socket (TestWeb.Router)
    (test 0.1.0) lib/phoenix/router.ex:406: TestWeb.Router.call/2
    ...

Obviously, I’m missing something trivial but fundamental here, but I can’t figure out what it is.
Please help.