MERN App getting connection error in digitalocean - node.js

Question

I have created my Blog using MERN application.
So now I am hosting this in digitalocean server but I am getting ECONNREFUSED error.
I have followed most of the document in both youtube and other for hosting it.
I can see my client running on ex: http://154.23.54.44:3000 but it is not running without :3000 as well as neither connecting my mlab conection.
Any suggestion for solving this and running this aplication in real production
Many Thanks.
//server package.json
"main": "server.js",
"scripts": {
"client-install": "npm install --prefix client",
"start": "node server.js",
"server": "nodemon server.js",
"client": "npm start --prefix client",
"dev": "concurrently \"npm run server\" \" npm run client\"",
"heroku-postbuild": "NPM_CONFIG_PRODUCTION=false npm install --prefix client && npm run build --prefix client"
}
//server.js
const express = require('express');
const mongoose = require('mongoose');
const path = require('path');
const config = require('config');
const app = express();
//bodyParser middleware
app.use(express.json());
//DB config
const db = config.get('mongoURI');
//connect to mongo
mongoose
.connect(db, {useNewUrlParser: true, useUnifiedTopology: true, useCreateIndex: true})
.then(() => console.log('MongoDB connected...'))
.catch(err => console.log(err));
//use routes
app.use('/api/users', require('./routes/api/users'));
app.use('/api/auth', require('./routes/api/auth'));
// Serve static assets if in production
if(process.env.NODE_ENV === 'production') {
// SET static folder
app.use(express.static('client/build'));
app.get('*', (req, res) => {
res.sendFile(path.resolve(__dirname, 'client', 'build', 'index.html'));
});
}
const port = process.env.PORT || 80;
app.listen(port, () => console.log(`Server started on PORT ${port}`));
//client (package.json)
{
"name": "client",
"version": "0.1.0",
"private": true,
"dependencies": {
"#testing-library/jest-dom": "^4.2.4",
"#testing-library/react": "^9.3.2",
"#testing-library/user-event": "^7.1.2",
"axios": "^0.19.2",
"bootstrap": "^4.4.1",
"bootstrap-less": "^3.3.8",
"moment": "^2.24.0",
"node-sass": "^4.13.1",
"react": "^16.13.1",
"react-dom": "^16.13.1"
},
"scripts": {
"start": "react-scripts start",
"build": "react-scripts build",
"test": "react-scripts test",
"eject": "react-scripts eject"
},
"eslintConfig": {
"extends": "react-app"
},
"browserslist": {
"production": [
">0.2%",
"not dead",
"not op_mini all"
],
"development": [
"last 1 chrome version",
"last 1 firefox version",
"last 1 safari version"
]
},
"proxy": "http://localhost:5000"
}
//error on production