Pagination not working Search posts

Question

if ( have_posts() ) {
  while ( have_posts() ) {
    the_post();
    global $post;
    //loop here
  }
  if(function_exists('wp_pagenavi')) { wp_pagenavi(); }
}

I have the following code in function as per the previous answer Pagination not working for category posts

add_action( 'pre_get_posts', function ( $q )
{
    if (    !is_admin() // Only target the front end queries
         && $q->is_main_query() // Targets the main query only
         && $q->is_category() // Only target category pages
    ) {
        $q->set( 'posts_per_page', 10 );
        $q->set( 'meta_key', 'custom_key' );
        $q->set( 'meta_value', '' );
        $q->set( 'meta_compare', '!=' );
        $q->set( 'orderby', 'meta_value_num' );
        $q->set( 'ignore_sticky_posts', 1 );
    }
});