Pagination on Custom Page not working

I have made a page-portfolio.php within two loops:

<?php
/**
 * Template Name: Portfolio
 * The template for displaying portfolio posts.
 *
 * @package Theme Name
 */

get_header(); ?>
<div id="content" class="site-content">
	<div id="primary" class="content-area">
		<main id="main" class="site-main" role="main">

                    <?php while ( have_posts() ) : the_post(); ?>

                            <?php get_template_part( 'content', 'page' ); ?>

                            <?php if ( comments_open() || get_comments_number() ) : ?>
                                    <section class="section-gradient">
                                        <div class="container">
                                            comments_template();
                                        </div><!-- .container -->
                                    </section>
                            <?php endif; ?>

                    <?php endwhile; // end of the loop. ?>
                    <?php wp_reset_postdata(); ?>

            <section class="section-gradient">
                <div class="container">
                    <div class="row">
				        <?php
                        $paged = ( get_query_var('page') ) ? get_query_var('page') : 1;
                        $args = array( 'post_type' => 'portfolio', 'paged' => $paged, 'posts_per_page' => 3 );
                        $loop = new WP_Query( $args );
                        while ( $loop->have_posts() ) : $loop->the_post(); ?>

                            <?php get_template_part( 'content', 'portfolio' ); ?>

                            <?php if ( comments_open() || get_comments_number() ) : ?>
                                    <section class="section-gradient">
                                        <div class="container">
                                            comments_template();
                                        </div><!-- .container -->
                                    </section>
                            <?php endif; ?>

                        <?php endwhile; // end of the loop. ?>
                        <?php wp_reset_postdata(); ?>
                    </div><!-- .row -->
                </div>
            </section>
            <section class="section-lightdark">
                <div class="container">
                <?php
                  if ( function_exists('wp_bootstrap_pagination') )
                    wp_bootstrap_pagination();
                ?>
                </div>
            </section>
		</main><!-- #main -->
	</div><!-- #primary -->
<?php get_footer(); ?>

I’m using wp_bootstrap_pagination from here: github

On may Archive Pages, the navigation works, but onlay there… what is wrong?