Access Denied Error when Connecting Node.js App to MySQL RDS Instance

I'm working on a Node.js application using mysql2/promise to connect to an Amazon RDS MySQL instance. Despite having correct credentials and network configurations, I'm encountering an Access denied error. Here are the details:

Error Message:

Error: Access denied for user 'db_user'@'my.ip.address' (using password: YES)

Connection Code:

const mysql = require('mysql2/promise');
const connection = await mysql.createConnection({
  host: process.env.DB_HOST,
  user: process.env.DB_USER,
  password: process.env.DB_PASSWORD,
  database: process.env.DB_NAME,
  port: process.env.DB_PORT
});

Environment Variables:

DB_HOST=mydatabase.example.com
DB_PORT=3306
DB_USER=db_user
DB_PASSWORD=hidden_password
DB_NAME=my_database

Security Groups: Configured to allow my local IP address.

I've verified the credentials and user privileges but still face this issue. Any insights or suggestions?

A bit more info of what I've tried as well:

GRANT ALL PRIVILEGES ON admin.* TO 'db_user'@'my.ip.address'; FLUSH PRIVILEGES;

While connected to the RDS instance locally through the workbench.

That resulted in

Access Denied

This is just a sample of one of my Express routes:

    const express = require('express');
    const dotenv = require('dotenv');
    const path = require('path');
    const mysql = require('mysql2/promise');
    const router = express.Router();
    const bcrypt = require('bcryptjs');
    const authenticateToken = require('./middleware/authenticateToken.cjs');
    
    const env = process.env.NODE_ENV || 'development';
    dotenv.config({ path: path.resolve(__dirname, '../.env') });
    
    var pool = mysql.createPool({
        host: process.env.DB_HOST,
        user: process.env.DB_USER,
        password: process.env.DB_PASSWORD,
        database: process.env.DB_NAME,
        port: process.env.DB_PORT
      });
    
    // Handling connection establishment
    pool.getConnection((err, connection) => {
        if (err) {
          console.error('Database connection error:', err);
          return;
        }
        console.log('Database connection established');
        connection.release(); // Release the connection back to the pool
      });
    
    router.get('/get-locations', authenticateToken, async (req, res) => {
      try {
        const [results] = await pool.query('SELECT * FROM location');
        res.status(200).json(results);
      } catch (error) {
        console.error('Error fetching locations:', error);
        res.status(500).send('Error fetching locations');
      }
    });