Thanks to @Buttered_Toast i’ve managed to get it work by changing:
<?php the_posts_pagination( array(
'mid_size' => 2,
'prev_text' => __( 'Back', 'textdomain' ),
'next_text' => __( 'Onward', 'textdomain' ),
) ); ?>
to:
<?php echo paginate_links( array(
'base' => str_replace( 999999999, '%#%', esc_url( get_pagenum_link( 999999999 ) ) ),
'total' => $postslist->max_num_pages,
'current' => max( 1, get_query_var( 'paged' ) ),
'format' => '?paged=%#%',
'show_all' => false,
'type' => 'plain',
'end_size' => 2,
'mid_size' => 1,
'prev_next' => true,
'prev_text' => sprintf( __( '<', 'text-domain' ) ),
'next_text' => sprintf( __( '>', 'text-domain' ) ),
'add_args' => false,
'add_fragment' => '',
) );
wp_reset_postdata(); ?>