Wordpress: Split the single page for each array without "<!--nextpage-->" on the template. Supported the preview page.

functions.php

<?php
function is_single_paged($page){
	global $wp_query;
	if(!is_preview()){
		$paged = (get_query_var('page')) ? get_query_var('page') : 1;
	}else{
		$pagenum = $_GET['paged'];
		$paged   = ($pagenum) ? $pagenum : 1;
		$wp_query->query = array('p' => $post->ID, 'page' => $paged, 'preview' => true, 'post_type' => get_post_type() );
		$wp_query->set('paged', $paged);
	}
	if(is_single() && $paged == $page){
		return true;
	}else{
		return false;
	}
}
?>

single.php

<?php get_header(); ?>

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

<article>
	<h1><?php the_title(); ?></h1>
	<?php if (is_single_paged(1)): ?>
	<section>
		something...
	</section>
	<?php endif; ?>

	<?php
		$fields    = CFS()->get('section');
		$fields    = array_chunk($fields, 2);
		if(!is_preview()){
			$paged   = (get_query_var('page')) ? get_query_var('page') : 1;
		}else{
			$pagenum = $_GET['paged'];
			$paged   = ($pagenum) ? $pagenum : 1;
		}
		$key       = $paged - 1;    // "-1" For Array's key
		$pagecount = count($fields);
		$fields    = $fields[$key];
		foreach ($fields as $field):
	?>
		<section>
			<?php if ($field['h2']): ?>
				<h2><?php echo $field['h2']; ?></h2>
			<?php endif; ?>

			<?php if ($field['h3']): ?>
				<h3><?php echo $field['h3']; ?></h3>
			<?php endif; ?>

			<?php if ($field['text']): ?>
				<div class="wp-editor">
					<?php echo $field['text']; ?>
				</div>
			<?php endif; ?>
		</section>
	<?php endforeach; ?>

	<section class="pager">
		<?php
			if(!is_preview()){
				$args1 = array(
					'base'           => get_the_permalink() . '%#%/',
					'format'         => get_the_permalink() . '%#%/'
				);
			}else{
				$args1 = array(
					'base'           => get_the_permalink() . '&paged=%#%',
					'format'         => get_the_permalink() . '&paged=%#%'
				);
			}
			$args2 = array(
				'total'              => $pagecount,
				'current'            => $paged,
				'show_all'           => false,
				'end_size'           => 1,
				'mid_size'           => 2,
				'prev_next'          => true,
				'prev_text'          => __('&laquo; Previous'),
				'next_text'          => __('Next &raquo;'),
				'type'               => 'list',
				'add_args'           => false,
				'add_fragment'       => '',
				'before_page_number' => '',
				'after_page_number'  => ''
			);
			$args = array_merge($args1, $args2);
			echo paginate_links($args);
		?>
	</section>
</article>

<?php endwhile; endif; ?>

<?php get_footer(); ?>