So far, I have found such a quick solution: in the template, the file content-page.php find the
<?PHP strings if( has_post_thumbnail() ): ?>
<header class="entry-header">
<?php if (the_post_thumbnail() ); ?>
</header>
<?php endif; ?>
Replace with this code
<? php if (has_post_thumbnail ()):? >
<header class="entry-header">
<?php if ( !is_single() || the_post_thumbnail() ); ?>
</header>
<?php endif; ?>
But I would also like to clarify with the author of the template, is this option possible or will you need to edit something else?