Express.js issues Router not working with phusion passenger Node.js

I have problems with Phusion Passenger and Express.js the router does not work and does not give me any kind of error

Try on a hosting with Node versions 16 and 14, as well as on a dedicated server with Node 16 (Cent OS)

Express version is: 4.18.2

const express = require("express");

const app = express();

const  router = express.Router();

// domain.com/backend/

app.use('/backend/test', (req, res) => {
  res.json({ message: 'test' })
})


app.get("/backend/test2", ({req, res}) => {
    res.json({ message: "test2" }) 
})

router.get("/backend/testrouter", ({req, res}) => {
    res.json({ message: "testrouter" }) 
})

router.get("/backend/testrouter2", ({req, res}) => {
  res.json({ message: "testrouter2" }) 
})

app.use((req, res) => {
  res.status(404).json({ message: 'error' });
});

app.listen(3000);

The first two endpoints work, while the last two do not work

You should use router as a middleware for app:

const express = require("express");
const app = express();
const  router = express.Router();

// domain.com/backend/

app.use('/backend/test', (req, res) => {
  res.json({ message: 'test' })
})

app.get("/backend/test2", ({req, res}) => {
    res.json({ message: "test2" }) 
})

router.get("/backend/testrouter", ({req, res}) => {
    res.json({ message: "testrouter" }) 
})

router.get("/backend/testrouter2", ({req, res}) => {
  res.json({ message: "testrouter2" }) 
})

app.use(router) // <== add this line

app.use((req, res) => {
  res.status(404).json({ message: 'error' });
});

app.listen(3000);