Saving the model to the database only works if the model (mongoose) is in the same file, I don't understand why? The error that occurs when importing a model from another file is as follows:

MongooseError: Operation users.insertOne() buffering timed out after 10000ms at Timeout. (/var/www/bhp_2/server/node_modules/mongoose/lib/drivers/node-mongodb-native/collection.js:149:23)

• mode: v16.13.0
• mongodb: 4.1.4
• mongoose: 6.0.12

connection.ts

import { Mongoose } from "mongoose"

//I also tried with es6 import like above
const User = require('../models/users')    

const mongoose = new Mongoose()
const conectionOpt = {
    useNewUrlParser: true
}     

const mongoDbUri = process.env.MONGODB_URI   

mongoose.connect(mongoDbUri, conectionOpt).then(()=>{
  console.log("MongoDB database connection established successfully");

// if I uncomment the code snippet below and comment out the import model file at the top, then insert to the database will work without a problem
//
//   const userShema = new mongoose.Schema({
//   first_name: {
//       type: String,
//       required: true,
//       trim: true
//   },
//   last_name: {
//       type: String,
//       required: true,
//       trim: true
//   }
// })

// const User = mongoose.model('User', userShema)

  const user = new User({
    "first_name" : "Bob",
    "last_name": "Test"
  })

  user.save();
}).catch(err => console.log(err))

users.ts

import mongoose from "mongoose";

const userShema = new mongoose.Schema({
    first_name: {
        type: String,
        required: true,
        trim: true
    },
    last_name: {
        type: String,
        required: true,
        trim: true
    }
})

const User = mongoose.model('User', userShema)
module.exports = User