getting 404 on my Express app after deployment - node.js

Question

I have an app that run normally in local development but when I deploy it (Ubuntu 21.x server) I run node/pm2 start (even installed nodemon to try) I get 404 error(not my 404 page). That how I deployed it after logging to server:
sudo apt update
sudo apt upgrade
curl -sL https://deb.nodesource.com/setup_14.x | sudo -E bash -
sudo apt-get install -y nodejs
git clone (repo)
cd (repo)
npm install
then I run it with
node app.js
//or
pm2 start app.js
//even using nodemon
nodemon app.js
and I installed Nginx and I get their welcoming HTML when I refer to IP address but I get 404 when I go the project port 3000
that's my server code except the POST req:
const express = require('express');
const helmet = require('helmet');
const compression = require('compression');
const bodyParser = require('body-parser');
// express app
const app = express();
app.use(helmet);
app.use(compression());
// bodyparser middleware
app.use(bodyParser.urlencoded({ extended: false }));
app.use(bodyParser.json());
// listen for request
app.listen(process.env.PORT || 3000);
// miidleware static files
app.use(express.static('public'));
app.use(express.static('script'));
app.get('/', (req, res) => {
res.sendFile('./views/index.html', { root: __dirname });
});
app.use((req, res) => {
res.status(404).sendFile('./views/404.html', { root: __dirname });
});
spent around 12 days trying so would appreciate help very much.

Answer 1

It seems that the server you have deployed on has an open port 80, but not port 3000.
If you want to access your express app via port 3000, you need to open the port on the Ubuntu server. How to do this depends on a vast number of variables from where the server is located to what firewall you are using.
If what you are trying to do is have your express app shown by Nginx on port 80, you will need to create a reverse proxy using Nginx.
To do this, read up on this here. There are too many variables for us to help further