How Can I Fix This Code So The Next/Previous Buttons Show Posts From Same "Tag"?

function get_next_prev_posts() {
        if (!$this->is_single) {
            return '';
        }

        if (td_util::get_option('tds_show_next_prev') == 'hide') {
            return '';
        }

        $buffy = '';

        $next_post = get_next_post();
        $prev_post = get_previous_post();

        if (!empty($next_post) or !empty($prev_post)) {
            $buffy .= '<div class="td-block-row td-post-next-prev">';
            if (!empty($prev_post)) {
                $buffy .= '<div class="td-block-span6 td-post-prev-post">';
                $buffy .= '<div class="td-post-next-prev-content"><span>' .__td('Previous article', TD_THEME_NAME) . '</span>';
                $buffy .= '<a>ID)) . '">' . get_the_title( $prev_post->ID ) . '</a>';
                $buffy .= '</div>';
                $buffy .= '</div>';
            } else {
                $buffy .= '<div class="td-block-span6 td-post-prev-post">';
                $buffy .= '</div>';
            }
            $buffy .= '<div class="td-next-prev-separator"></div>';
            if (!empty($next_post)) {
                $buffy .= '<div class="td-block-span6 td-post-next-post">';
                $buffy .= '<div class="td-post-next-prev-content"><span>' .__td('Next article', TD_THEME_NAME) . '</span>';
                $buffy .= '<a>ID)) . '">' . get_the_title( $next_post->ID ) . '</a>';
                $buffy .= '</div>';
                $buffy .= '</div>';
            }
            $buffy .= '</div>'; //end fluid row
        }

        return $buffy;
    }

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