Connection to express websocket server

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1

Hey I have an issue contecting my expo app by websockets with my webserver.

First I tested my websockets of my app with WebSockets - 👩‍💻 Do tests for FREE 💪 - SocketsBay.com and they work.
After that I implemented an express server. And I can get a connection with postman through the local address ws://127.0.0.1:4000/

My problem: I cannot connect to the websocket server with my expo app.
I think the problem is, that I cannot take the localhost ip address, since the app is running on my phone. So I have to use the local ip adress within the local wifi-network of my laptop. But this is also not working. I don’t know why. Hope someone can help me.

My websocket server is very basic:

import path from 'path'
import { fileURLToPath } from 'url'

const __filename = fileURLToPath(import.meta.url);
const __dirname = path.dirname(__filename);

import express from 'express'
import expressWs from 'express-ws'
import http from 'http'

let port = 4000;

let app = express();
let server = http.createServer(app);

expressWs(app, server);
app.use(express.static(__dirname + '/views'));

app.get('/', (req, res) => {
    console.log("Request on '/'")
    res.status(200).send("Welcome to our app");
});

app.ws('/', async function (ws, req) {
    console.log("New websocket connection")
    ws.send("Got connected to server.")
    ws.on('message', async function (msg) {
        console.log("Got new message: ")
        console.log(msg); //this only gives me and output when I send with postman, not with my app
    });
});

server.listen(port, () => {
    console.log("Server runs on port %s...", port)
})