Inserting php DOM element via for loop

Question

I am attempting to insert a new dom element after every 5 p tags but I cant get it to perform the insert. It is inserting at the end of the document. The page I am inserting into has 50+ p tags so I am not sure what I am doing wrong. Any help would be great. Here is what I am using:

$body = $this->get_body_node();

// Build our amp-ad tag
$ad_node = AMP_DOM_Utils::create_node( $this->dom, 'amp-ad', array(
    'width' => 300,
    'height' => 250,
    'type' => 'ad',
    'data-ad-client' => '###',
    'data-ad-slot' => '###'
) );

// Add a placeholder to show while loading
$fallback_node = AMP_DOM_Utils::create_node( $this->dom, 'amp-img', array(
    'placeholder' => '',
    'layout' => 'fill',
    'src' => 'https://placehold.it/300X250',
) );
$ad_node->appendChild( $fallback_node );

$p_nodes = $body->getElementsByTagName( 'p' );
$pListLength = $p_nodes->length;
if ( $pListLength > 5 ) {
    for($i=5; $i <= $pListLength; $i+=5) {
        $p_nodes->item($i)->parentNode->insertBefore( $ad_node, $p_nodes->item($i));
    }
}