A few lines of code to manually test a WordPress database connection. Add below DB define's in your wp-config.php

manually-test-wordpress-database-connection.php

/**
 * Manually test WordPress database connection. Return useful errors upon failure.
 */
$mysqli = new mysqli(DB_HOST, DB_USER, DB_PASSWORD, DB_NAME);
if ($mysqli->connect_errno) {
  die("❌ Failed to connect to MySQL: {$mysqli->connect_error}");
}
if ($result = $mysqli->query("SELECT * FROM `{$table_prefix}options`")) {
  echo "✅ Connection OK. Returned rows are: {$result->num_rows}";
  $result->free_result();
}
$mysqli->close();