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Hi,
I have checked your site on our Pc and see that it work fine.
See the screenshot: http://prntscr.com/8j40y0
If you want to make the navigation on the mobile as your image mentioned in here, you can do as the following:
Add the following code to the Custom CSS:

@media (max-width: 767px){
.nav-previous {
    float: left;
    width: 50%;
}
.nav-next {
    float: left;
    width: 50%;
    text-align: right;

}
.post-navigation .nav-links .nav-next {
    margin-top: 0;
}
.post-navigation .nav-links .nav-next::before, .post-navigation .nav-links .nav-previous::before {
    content: none;
}
.post-navigation .nav-links .nav-next a, .post-navigation .nav-links .nav-previous a  {
    background-color: #000;
    padding: 6px 11px;
    color: #fff;
    font-size: 20px;
}
}

Finally, open the single.php file. Find the line 35:
Replace the following code:
<?php the_post_navigation(); ?>

With new code:

<nav class="navigation post-navigation">
    <div class="nav-links visible-xs">
    <div class="nav-previous">
       <?php previous_post('%', '« Previous', 'no'); ?>
    </div>      

        <div class="nav-next">
           <?php next_post('%', 'Next »', 'no'); ?>
    </div>      
      </div>
</nav>
<div class="hidden-xs">
    <?php the_post_navigation(); ?>
</div>

Hope this helps !