Hi,
this is possible but requires some rather advanced programming knowledge, you would need to start with creating a child template file for wpadverts/templates/list-item.php file as explained here https://wpadverts.com/doc/child-themes-and-templates/
Once you have a custom list-item.php file you would need to put inside it code that will display custom code every 5 ads, something like this
<?php
include ADVERTS_PATH . '/templates/list-item.php';
global $adverts_list_index;
if( $adverts_list_index < 1 ) {
$adverts_list_index = 0;
}
$adverts_list_index++;
if( $adverts_list_index % 5 === 0 ) {
$cl = adverts_css_classes( 'advert-item advert-item-col-'.(int)$columns, get_the_ID() );
echo sprintf( "<div class='%s'>[YOUR AD HERE]</div>", $cl );
}