Websocket client can't connect to the server on DigitalOcean app platform

I have a Nuxt 3 application with socket.io running on DigitalOcean app platform where the client can’t connect to the web socket server.

The web socket runs on the same port 3000 as the application, with:

• nuxt.config.js

import startSocketServer from "./server/sockets"

export default defineNuxtConfig({
  ...
  hooks: {
    listen: (nuxtServer) => startSocketServer(nuxtServer)
  },
  ...
});

• ./server/sockets/index.js

import {  Server } from "socket.io";

export default (nuxtServer) => {
  const io = new Server(nuxtServer)
  io.on("connection", (socket) => {
    console.log(`connection ${socket.id}`)
  });
};

And on the client I have:

const socketDomain = (isLocal) ? 'localhost:3000' : 'my.domain.com'

const socket = io(socketDomain, {
  transports: ['websocket'],

socket.on("connect", () => {
  console.log('connect')
});

socket.on("connect_error", (err) => {
  console.log(`connect_error due to ${err}`);
});

The application run perfectly locally with nuxi dev or nuxi build && node .output/server/index.mjs but when I deploy on DigitalOcean I get the following:

WebSocket connection to 'wss://my.domain.com/socket.io/?EIO=4&transport=websocket' failed:
doOpen @ entry.3f5d167e.js:21
open @ entry.3f5d167e.js:20
open @ entry.3f5d167e.js:21
$i @ entry.3f5d167e.js:21
A1.exports @ entry.3f5d167e.js:21
open @ entry.3f5d167e.js:21
(anonymous) @ entry.3f5d167e.js:21
invoice.22658294.js:1 connect_error due to Error: websocket error

I also tried with the web socket on a different port, but from my understanding it is not supported on DO.

I also tried with transports: ['polling', 'websocket'] but I got xhr poll error.

What could be the issue?