api/index.js

app.get('/api', (req, res) => {
    res.send("Hello from Node API server!");
});

app.get('/api/hello', (req, res) => {
    res.send("Hello!");
});

app.use("/api/v1/auth", authRoutes)
app.use("/api/v1/user", userRoutes)
app.use("/api/v1/product", productRoutes)
app.use("/api/v1/order", orderRoutes)

app.listen(port, () => {
    connectDB()
    console.log(`${chalk.green('✓')} ${chalk.blue(
        `Listening on port ${port}. Visit http://localhost:${port}/ in your browser.`
    )}`)
})

db/connect.js

import mongoose from "mongoose"

export const connectDB = async() => {
    try{
        console.log(process.env.MONGO_URI)
        const conn = await mongoose.connect(process.env.MONGO_URI)
        console.log(`MongoDB Connected: ${conn.connection.host}`);
    } catch(error){
        console.error(`Error: ${error.message}`)
        process.exit(1)
    }
}

api/routes/productRoute.js

router.get('/', getAllProducts)

api/controllers/productController.js

export const getAllProducts = async(req, res) => {
    try{ 
        const products = await Product.find({})
        console.log(products)

        res.status(200).send({
            success: true,
            products
        })
    } catch(error){
        res.status(500).send({
            success: false,
            message: "Error in fetching products!",
            error 
        })
        console.log(error)
    }
}

After I deployed the MERN Stack project to vercel, the api controllers could not read MongoDB data even I passed the MONGO_URI in environmental variables settings. It shows the error message MongooseError: Operation `products.find()` buffering timed out after 10000ms at Timeout.<anonymous> that I could not figure out since when running on local it could show out data.

I successfully deployed the app but the api still not working correctly so may need some help