Creating an Image gallery with Loop – Pagination

Hi there,
I am trying to create an image gallery for my own custom WP theme.
I am making use of a loop which shows me the thumbnails of the images found among the uploads. This is working fine, however I also wish to have a pagination function for the gallery.
The pagination nav shows up but when I click a link to page 2 or more, the page is not found (404).

(I am not a pro at coding…)
I have used this code (which is five years old) to get where I am at:

<?php
$paged = (get_query_var('paged')) ? get_query_var('paged') : 1;
$loopb = new WP_Query( array( 'post_type' => 'portfolio', 'posts_per_page' => 2, 'paged' => $paged ) ); ?>
<?php while ( $loopb->have_posts() ) : $loopb->the_post(); 

//display posts
?>
<?php endwhile; ?>
<?php
if($loopb->max_num_pages>1){?>
    <p class="navrechts">
    <?php
    for($i=1;$i<=$loopb->max_num_pages;$i++){?>
        <a href="<?php echo '?paged=' . $i; ?>" <?php echo ($paged==$i)? 'class="selected"':'';?>><?php echo $i;?></a>
        <?php
    }
    if($paged!=$loopb->max_num_pages){?>
        <a href="<?php echo '?paged=' . $i; //next link ?>">></a>
    <?php } ?>
    </p>
<?php } ?>

(This code is taken from here: https://wordpress.org/support/topic/adding-pagination-to-a-wp_query-loop#post-1516156 )

Can anyone tell me, why the pagination does not work properly?
Thanks for your time. Foolix