The problem is in this part
<?php if ( is_search() ) : // Only display Excerpts for Search ?>
<div class="entry-summary">
<div class="entry">
<?php $content = get_the_content();
$content = strip_tags($content);
echo '<p>' . substr($content, 0, 255) . ' [...] </p>';
?>
<hr class="post-divider"/>
</div>
</div>
<?php else : ?>
You need to add a link somewhere in there, like so:
<?php if ( is_search() ) : // Only display Excerpts for Search ?>
<div class="entry-summary">
<div class="entry">
<?php $content = get_the_content();
$content = strip_tags($content);
echo '<p>' . substr($content, 0, 255) . ' [...] </p>';
?>
<a href="https://wordpress.stackexchange.com/questions/327316/<?php echo get_the_permalink(); ?>">Read More</a>
<hr class="post-divider"/>
</div>
</div>
<?php else : ?>
Then you can modify the location and content of that link as you like.