Fatal error: require_once(): Failed opening required ... Novice programmer, appreciate any guidance - server

Question

I'm a novice in server-side programming so any advice is appreciated.
I have some PHP files uploaded on a virtual server (000webhost.com) and I'm having trouble with locating a file I'm requiring in another PHP file. The register.php is attempting to "require_once" the update_user_info.php
Here's the code for register.php (Which I've taken from a tutorial and adjusted it with my variables):
<?php
define('__ROOT__', dirname(dirname(__FILE__)));
require_once(__ROOT__.'../update_user_info.php');
$db = new update_user_info();
// json response array
$response = array("error" => FALSE);
if (isset($_POST['name']) && isset($_POST['email']) && isset($_POST ['password']) && isset($_POST['branch']) && isset($_POST['gender'])) {
// receiving the post params
$username = $_POST['username'];
$name = $_POST['name'];
$email = $_POST['email'];
$password = $_POST['password'];
$branch = $_POST['branch'];
$gender = $_POST['gender'];
// check if user is already existed with the same email
if ($db->CheckExistingUser($email)) {
// user already existed
$response["error"] = TRUE;
$response["error_msg"] = "User already existed with " . $email;
echo json_encode($response);
} else {
// create a new user
$user = $db->StoreUserInfo($username, $name, $email, $password, $branch, $gender);
if ($user) {
// user stored successfully
$response["error"] = FALSE;
$response["user"]["username"] = $user["username"];
$response["user"]["name"] = $user["name"];
$response["user"]["email"] = $user["email"];
$response["user"]["branch"] = $user["branch"];
$response["user"]["gender"] = $user["gender"];
echo json_encode($response);
} else {
// user failed to store
$response["error"] = TRUE;
$response["error_msg"] = "Unknown error occurred in registration!";
echo json_encode($response);
}
}
} else {
$response["error"] = TRUE;
$response["error_msg"] = "Required parameter (username, name, email or password) is missing!";
echo json_encode($response);
}
?>
And here's a screenshot of the error that's occurring when attempting to visit the link containing the register.php:
Screenshot of the error
What I've tried so far:
1- tried to use require_once __DIR__."/../filename.php"; and lots more of the same, basically tinkering the statement with different language constructs and constants.
2-Read tutorials on the correct way to use require once but their cases is almost always very different. Either they are using WAMP/XAMPP -meaning they have a local path not a virtual one- So no luck there.
3- tried to change .htaccess file, as someone said in a post that it solved his problem.
4- I'm certain that both files are uploaded to the same directory. Yet it says "No such file".
Does anyone have an idea of where the error could be stemming from? Thanks in advance!