next/previous post only in the same category?

I found several threads in the forum, but no solution works for me. I put this code snippet in the function.php of my child theme.

add_filter('avia_post_nav_categories', 'use_same_category_filter');
function use_same_category_filter($same_category)
{
$same_category = true;
return $same_category;
}

but nothing happened.
( from this post: https://kriesi.at/support/topic/nextprevious-post-arrow-same-category-content-only/ )