nsert ads after second paragraph of single post content.

add-ads-after-2nd-para.php

<?php
//Insert ads after second paragraph of single post content.
add_filter( 'the_content', 'wsm_insert_post_ads' );
function wsm_insert_post_ads( $content ) {

	$ad_code = '<div class="content-ad-block"><script async src="//pagead2.googlesyndication.com/pagead/js/adsbygoogle.js"></script>
<!-- Top Posts 300 250 -->
<ins class="adsbygoogle"
     style="display:inline-block;width:300px;height:250px"
     data-ad-client="ca-pub-2169025261524250"
     data-ad-slot="4828803481"></ins>
<script>
(adsbygoogle = window.adsbygoogle || []).push({});
</script></div>';

	if ( is_single() && ! is_admin() ) {
		return wsm_insert_after_paragraph( $ad_code, 2, $content );
	}

	return $content;
}

// Parent Function that makes the magic happen
 function wsm_insert_after_paragraph( $insertion, $paragraph_id, $content ) {
	$closing_p = '</p>';
	$paragraphs = explode( $closing_p, $content );
	foreach ($paragraphs as $index => $paragraph) {

		if ( trim( $paragraph ) ) {
			$paragraphs[$index] .= $closing_p;
		}

		if ( $paragraph_id == $index + 1 ) {
			$paragraphs[$index] .= $insertion;
		}
	}

	return implode( '', $paragraphs );
}

Borrowed from https://www.careerpointkenya.co.ke/ method of ad insertion.