adjacent post sort order by post title
<?php
$all_posts = new WP_Query(array(
'orderby' => 'menu_order',
'order' => 'ASC',
'posts_per_page' => -1
));
foreach($all_posts->posts as $key => $value) {
if($value->ID == $post->ID){
$nextID = $all_posts->posts[$key + 1]->ID;
$prevID = $all_posts->posts[$key - 1]->ID;
break;
}
}
?>
<?php if($prevID): ?>
<span class="prev">
<a href="<?= get_the_permalink($prevID) ?>" rel="prev"><?= get_the_title($prevID) ?></a>
</span>
<?php endif; ?>
<?php if($nextID): ?>
<span class="next">
<a href="<?= get_the_permalink($nextID) ?>" rel="next"><?= get_the_title($nextID) ?></a>
</span>
<?php endif; ?>