Wordpress CPT pagination returns 404 page - wordpress

Question

I want to add pagination to my CPT wordpress posts. I've seen some of the solutions, but neither I see no pagination or the pagination does not work.
I've added some pagination code, and it shows the correct number of pages but when I click on page number it returns 404 page. The url is also correct.
Here's my code:
<div class="container">
<div class="row">
<?php $paged = ( get_query_var( 'paged' ) ) ? get_query_var( 'paged' ) : 1; ?>
<?php $args = array( 'post_type' => 'Realizacje', 'posts_per_page' => 6, 'category_name' => array('projekty-wnetrz', 'wykonczenia-pod-klucz'), 'paged' => $paged); ?>
<?php $loop = new WP_Query($args); ?>
<?php $count=0; ?>
<?php if ( $loop->have_posts() ) : while ( $loop->have_posts() ) : $loop->the_post();
$num = $loop->post_count;
?>
<div class="col-md-4">
<div class="realisation">
<!-- <div class="gallery">
<?php
$images = get_field('galeria');
$size = 'medium';
if( $images ): ?>
<div id="lightgallery<?php echo $count; ?>">
<?php foreach( $images as $image ): ?>
<div class="gal">
<img src="<?php echo $image; ?>"/>
</div>
<?php endforeach; ?>
</div>
<?php endif; ?>
</div> -->
<div class="galeria">
<div class="lightg" id="lightgallery<?php echo $count; ?>">
<a href="<?php echo get_the_post_thumbnail_url(); ?>">
<div class="relative-box"><img class="first_image"src="<?php echo get_the_post_thumbnail_url(); ?>"/>
<div class="demo-gallery-poster">
<i class="fa fa-search" aria-hidden="true"></i>
</div></div>
<div class="gallery-title"><h5 class="gallery-t"><?php the_title(); ?></h5></div>
</a>
<?php
$count=$count+1;
$images = get_field('galeria');
$size = 'large';
if( $images ): ?>
<?php foreach( $images as $image ): ?>
<img src="<?php echo $image; ?>"/>
<?php endforeach; ?>
<?php endif; ?>
</div>
</div>
</div>
</div>
<?php endwhile; ?>
<?php else: ?>
<h5>Brak realizacji</h5>
<?php endif; ?>
<div class="pagination">
<?php
$big = 999999999;
echo paginate_links( array(
'base' => str_replace( $big, '%#%', get_pagenum_link( $big ) ),
'format' => '?paged=%#%',
'current' => max( 1, get_query_var('paged') ),
'total' => $loop->max_num_pages,
'prev_text' => '«',
'next_text' => '»'
) );
?>
</div>
<?php wp_reset_postdata(); ?>
</div>
</div>