Custom template not working with mobile

I created a child theme based on Twenty-Sixteen. Within the child theme, I created a file titled page.php. This is the code:

<?php

define('WP_USE_THEMES', false); //This code stops the theme files from loading

while (have_posts()) : the_post(); 
    the_content();
endwhile; 

echo "<br><br>";
echo "Page template in child theme.";

echo "<br><br>";
echo "<div id=postID>".get_the_ID()."</div>" ;

?>

It works perfectly on my laptop, but it doesn’t work on my iPhone. Why is that? How can I fix it?