Try using these codes:
if( isset( $sub_category ) ){
echo '<b>more items in: </b>' . $sub_category->name;
$args = array(
'cat' => $sub_category->term_id,
'post__not_in' => array( get_the_ID() )
);
$relatedpostsinsubcategory = new WP_Query( $args );
if( $relatedpostsinsubcategory->have_posts() ){
while( $relatedpostsinsubcategory->have_posts() ){
$relatedpostsinsubcategory->the_post();
?>
<a href="https://wordpress.stackexchange.com/questions/324478/<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>"><?php the_title(); ?></a>
<?php
}
wp_reset_postdata();
}
}
You can find more information about getting subcategories and related posts here.