How to host a nodeJS ReactJS app on Gandi (or other hosting company)?

For the first time, I don't use Heroku but I try to host my website (node and express for the back and react for the front) with Gandi. Unfortunately, so far I have a 503 error. Here is how I use to do on Heroku:

Server.js file

const express = require("express");
const mongoose = require("mongoose");
const bodyParser = require("body-parser");

const app = express();

// Body parser middleware
app.use(bodyParser.urlencoded({ extended: false }));
app.use(bodyParser.json());

const profiles = require("./routes/profiles");
const categories = require("./routes/categories");
const skills = require("./routes/skills");
const message = require("./routes/message");

// DB config
const db = require("./config/keys.js").mongoURI;

// Connect to mongoDB
mongoose
  .connect(
    db,
    { useNewUrlParser: true }
  )
  .then(() => console.log("MongoDB connected"))
  .catch(err => console.log(err));

// Use Routes
app.use("/profiles", profiles);
app.use("/categories", categories);
app.use("/skills", skills);
app.use("/message", message);

// Server 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 || 5000;

app.listen(port, () => console.log(`Server running on port ${port}`));

Package.json file:

{
  "name": "adopt-a-dev",
  "version": "1.0.0",
  "description": "Find the best dev for your company",
  "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"
  },
  "author": "Guillaume Cogito",
  "license": "MIT",
  "dependencies": {
    "body-parser": "^1.18.3",
    "concurrently": "^3.6.1",
    "express": "^4.16.3",
    "mongoose": "^5.2.7",
    "nodemailer": "^4.6.8",
    "validator": "^10.6.0"
  },
  "devDependencies": {
    "nodemon": "^1.18.3"
  }
}

It works well on heroku but I don't know what to change to make it works on
Gandi. It someone have some advice or maybe a good tuto to know where to start it would be nice. Thank you for your help.