set_role has no effect

I am running WordPress 4.7.5 and I am trying to set user roles programmatically for a customer. However set_role does not seem to have any effect.

The user is created, but the users role is subscriber. I also tried different roles, such as $user->set_role('administrator') – also to no avail.

Here is my code – what am I doing wrong?

require($_SERVER['DOCUMENT_ROOT'] . '/wp-load.php');
    if ( !username_exists( 'mytestuser' ) ) {
        $user_id = wp_create_user('mytestuser', 'testpass345','testusers@mailaccount.tk');
        $user = new WP_User( $user_id );
    $user->set_role( 'author' );
    }

Thanks in advance!

You can use wp_update_user to update the role for your newly created user.

if ( !username_exists( 'mytestuser' ) ) {
    $user_id = wp_create_user('mytestuser', 'testpass345','testusers@mailaccount.tk');
    $user_id = wp_update_user( array( 'ID' => $user_id, 'role' => 'author' ) );
}

Further reading at WordPress Codex.

Or, you can remove the current role and assign a new one:

if ( !username_exists( 'mytestuser' ) ) {
    $user_id = wp_create_user('mytestuser', 'testpass345','testusers@mailaccount.tk');
    $user = new WP_User( $user_id );
    $user->remove_role( 'subscriber' );
    $user->add_role( 'author' );
}