MyBB Community Forums - How to edit database user groups.

Hello, im Debugger. Ive created a payment system using sellix, and once the user has paid it redirects them to a webhook and runs a function but it dosent seem to change the users group, im not sure of as why, i also dont get any errors, here is a snippet of the code, it would be great if anyone could help me find a fix, or tell me if im doing it wrong.

Code:

<?php

define("IN_MYBB", 1);
require_once "./global.php";
$userN = $mybb->user['uid'];
$userG = $mybb->user['usergroup'];


//get the nottifcation from sellix
$payload = file_get_contents('php://input');


$secret = "no";

// get our signature header
$header_signature = $_SERVER["HTTP_X_SELLIX_SIGNATURE"];

//verify that notifcation came from sellix
$signature = hash_hmac('sha512', $payload, $secret);
if (hash_equals($signature, $header_signature)) {

    //decode the payload to json
    $jsonString = json_decode($payload, true);

    $event = $jsonString['event'];
    $data = $jsonString['data'];
    $uqid = $data['uniqid'];
    $status = $data['status'];


    if($status == 'COMPLETED'){


    $query = $db->query("UPDATE ".TABLE_PREFIX."users SET usergroup='3' WHERE uid='".$userN."'");



    }


}

?>