<?php
    if (is_single() && in_category('stories')) {
        $post_id = $post->ID;
        $cat = get_the_category(); //I've tried changing this to my category (both ID and slug)
        $current_cat_id = $cat[0]->cat_ID;  //Also tried plugging ID and slug

        $args = array(
            'category' => $current_cat_id, //Also tried plugging ID and slug
            'orderby' => 'post_date',
            'order' => 'DESC'
        );
        $posts = get_posts($args);

        $ids = array();
        foreach ($posts as $thepost) {
            $ids[] = $thepost->ID;
        }

        $thisindex = array_search($post_id, $ids);
        $previd = $ids[$thisindex - 1];
        $nextid = $ids[$thisindex + 1];

        if (!empty($nextid)) {
            ?><div class="double-grid"><a rel="next" href="<?php echo get_permalink($nextid) ?>"><div class="image-tile tile-on-archive-page" style="background-image: url('<?php echo get_the_post_thumbnail_url($nextid); ?>'"> <div class="gold-button">LAST STORY >></div></div></a></div><?php
        }
        if (!empty($previd)) {
            ?><div class="double-grid"><a rel="prev" href="<?php echo get_permalink($previd) ?>"><div class="image-tile tile-on-archive-page" style="background-image: url('<?php echo get_the_post_thumbnail_url($previd); ?>'"> <div class="gold-button">NEXT STORY >></div></div></a></div><?php
        }
    }
    ?>