Replace the “Main Content” block in index.php with the following. This way you do not need a content.php file and can query the posts right in index.php. The Next/Previous post navigation will be functional as-well.
<!-- Main Content -->
<div class="container">
<div class="row">
<div class="col-lg-8 col-lg-offset-2 col-md-10 col-md-offset-1">
<?php
// Start the loop.
while ( have_posts() ) : the_post();
?>
<div class="post-preview">
<a href="https://wordpress.stackexchange.com/questions/244653/<?php the_permalink(); ?>">
<h2 class="post-title">
<?php the_title(); ?>
</h2>
<h3 class="post-subtitle">
<?php the_excerpt(); ?>
</h3>
</a>
<p class="post-meta">Posted by <?php the_author(); ?> on <?php the_time('jS F Y') ?></p>
</div>
<?php
// End of the loop.
endwhile;
?>
<!-- Pager -->
<ul class="pager">
<li class="next">
<?php next_posts_link('« Older Entries') ?>
<?php previous_posts_link('Newer Entries »') ?>
</li>
</ul>
</div>
</div>
</div>