Loop not working on custom page

hello,
I have a loop in a custom page template. The loop works fine when i use it on the home page. But it doesn’t work when i use it on other page using the same template has home page.
The loop fetches posts from different category and displays on the page. I am using theme twenty thirteen( latest updated )
I am trying to do the same as in this
https://www.wpbeginner.com/wp-themes/how-to-create-a-grid-display-of-post-thumbnails-in-wordpress-themes/

Here is the code

<?php

global $query_string;	query_posts($query_string.'posts_per_page=12'.'post_type=post');
if(have_posts()) :	while(have_posts()) :  the_post();
?>