Operation `users.findOne()` buffering timed out after 10000ms when deploying MERN app to heroku

I'm having trouble when deploying my app to Heroku. I keep getting this error, but locally it works perfectly.

Maybe it has something to do with functions being called before mongo establishes the connection, but I don't know how to fix this (in case it is the problem)

Server.js

import express from 'express';
import mongoose from 'mongoose';
import dotenv from 'dotenv';
import path from 'path';

dotenv.config();

const app = express();
app.use(express.json());
app.use(express.urlencoded({ extended: true }));
mongoose.connect(`mongodb+srv://${process.env.DB_USER}:${process.env.DB_PASS}@name.0xzkc.mongodb.net/${process.env.DB_NAME}?retryWrites=true&w=majority&ssl=true`,{
  useNewUrlParser: true,
  useUnifiedTopology: true,
  useCreateIndex: true,
}).then(console.log('working'));

app.use('/api/uploads', uploadRouter);
app.use('/api/users', userRouter);
app.use('/api/products', productRouter);
app.use('/api/orders', orderRouter);

I really need some help here, I'll be really really really grateful