Set a page with slug "404-error" as your 404 page in WordPress

404.php

<?php get_header(); ?>
<div class="container-fluid">
	<div class="content">
		<main role='main'>
			<?php
			$query_404 = new WP_Query(array(
				'posts_per_page' => 1,
				'post_type' => 'page',
				'post_name' => '404-error'
			));
			while ( $query_404->have_posts() ) : $query_404->the_post();

				get_template_part( 'content', 'page' );

				if ( comments_open() || get_comments_number() ) :
					comments_template();
				endif;

			endwhile;
			wp_reset_postdata();
			?>
		</main>
	</div>
</div>
<?php get_footer(); ?>