Mongoose: Operation `schema.findOne()` buffering timed out after 10000ms - node.js

Question

I have a REST API made with Node.js and I use Mongoose to connect with MongoDB. Whenever I try to interact with the DB using Mongoose, I get the timeout error.
I have made sure that the DB is running and accepts connections from my IP. Using the same connection method in other projects works for some reason.
I am using mobile hotspot connection. Could that be an issue?
server.js
const express = require('express');
const app = express();
const cors = require('cors');
const userRoutes = require('../Backend/routes/userRoutes')
const taskRoutes = require('../Backend/routes/taskRoutes')
const mongoose = require('mongoose');
require('dotenv').config()
mongoose.connect(process.env.DB_CONNECTION);
app.use(cors());
app.use(express.json());
app.use(userRoutes)
app.use(taskRoutes)
app.listen(process.env.PORT || 3000 , () => {
console.log("API is now running.")
});
userRoutes.js
const userController = require('../controllers/userController')
const express = require('express');
const router = express.Router();
router.get('/users/currentUser', userController.getCurrentUserData)
router.get('/users/:userId', userController.getUserData)
router.post('/register', userController.register);
router.post('/login', userController.login);
module.exports = router;
Method in userController that causes the error (the error is caused by no matter which method I call out)
exports.login = async (req, res) => {
if (!req.body.username || !req.body.password) {
return res.status(400).send("Missing one or all required request body fields: username, password");
}
let existingUser;
try {
existingUser = await userSchema.findOne({ username: req.body.username });
} catch(err) {
return res.status(500).send("Internal server error."); //Error comes from here
}
if (!existingUser) {
return res.status(404).send('User not found');
}
const isPasswordCorrect = await bcrypt.compare(req.body.password, existingUser.password);
if (!isPasswordCorrect) {
return res.status(401).send('Invalid credentials');
}
let token;
try {
token = jwt.sign(
{ userId: existingUser._id, username: existingUser.username, tasks: existingUser.tasks},
process.env.SECRET,
{ expiresIn: "1h" }
);
} catch (err) {
console.log(err);
return res.status(500).send("Couldn't create JWT.")
}
return res.status(200).json({ token: token });
}
User model
const mongoose = require('mongoose');
const bcrypt = require('bcrypt');
const userSchema = new mongoose.Schema({
username:{
type: String,
unique: true
},
password:{
type: String
},
tasks:{
type:[mongoose.Types.ObjectId]
}
})
userSchema.pre('save', async function(next) {
const salt = await bcrypt.genSalt();
this.password = await bcrypt.hash(this.password, salt);
next();
});
module.exports = mongoose.model('user',userSchema);
.env
DB_CONNECTION="mongodb://localhost:27017/test"
SECRET='test'