AWS Lightsail Wordpress - connect to database on instance using mysqli

I have a wordpress website hosted on AWS Lightsail. On that website, I would like to use a shortcode to retrieve data from a database using mysqli. However, everytime the script that connects to the database is executed, I receive a connection error. The database is on the same AWS Lightsail instance. I was wondering if I need to establish an SSH-tunnel because this is also needed if I connect to phpmyadmin. But since the script is executed on the server itself this does not really makes sense in my understanding. I have used the exact same code on a local host and it worked perfectly fine. What am I missing here?

//Params to connect to a database
$dbHost = *static ip of wordpress site*;
$dbUser = root;
$dbPassword = *password*;
$dbName = *database name*;


mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT);
try {
  $db = new mysqli($dbHost, $dbUser, $dbPassword, $dbName);
  $db-set_charset(utf8mb4);
} catch(Exception $e) {
  error_log($e-getMessage());
  exit('Error connecting to database'); 
}