Socket.io does not run as expected on Heroku

everyone. I'm having an issue with Socket.io on Heroku. Everything runs wonderfully on my local machine, which means: I can see when a user makes an entry and it arrives just-in-time => both clients can see the message real-live! However, it doesn't work on Heroku. A client doesn't see whether the other party makes an input, let alone the message comes just-in-time. Only after refreshing the page. Here is my code:

This is my server code:

 const PORT = 2900;

 const server = http.createServer(app);
 const socketio = new Server(server, { cors: { origin: "*" } });


  socketio.on("connect", (socket) => {
  socket.on("addUser", (userId) => {
  addUser(userId, socket.id);

//socketio.emit("getUsers", users);
});

//tipping
socket.on("typing", ({ receiverId, senderId, text, tippt }) => {
const user = getUser(receiverId);

if (user)
  socketio.to(user.socketId).emit("userIsTyping", {
    senderId,
    text,
    tippt,
   });
});

//send and get message
socket.on("sendMessage", ({ receiverId, senderId, text }) => {
const user = getUser(receiverId);

if (user)
  socketio.to(user.socketId).emit("getMessage", {
    senderId,
    text,
  });
});

//when disconnect
socket.on("disconnect", () => {
removeUser(socket.id);
socketio.emit("getUsers", users);
 });
});


server.listen(PORT, () => console.log(`Listening on port ${PORT}`));

And that's the client code:

const socket = useRef();

useEffect(() => {
socket.current = io(":2900");

socket.current.on("getMessage", (data) => {
  setArrivalMessage({
    sender: data.senderId,
    text: data.text,
    createdAt: Date.now(),
  });
});
socket.current.on("userIsTyping", (data) => {
  setGibtEin(data.tippt);
  setAnUser(data.senderId);
});
}, []);