functions.php

add_filter( 'the_content', 'prefix_insert_post_ads' );
/**
 * Insert code for ads after second paragraph of single post content.
 *
 * @param  string $content Post content
 *
 * @return string          Amended content
 */
function prefix_insert_post_ads( $content ) {
	$content_callout = '<h4>Insert code for ads after second paragraph of single post content.</h4>';

	if ( is_single() && ! is_admin() ) {
		return prefix_insert_after_paragraph( $content_callout, 2, $content );
	}
	return $content;
}

/**
 * Insert something after a specific paragraph in some content.
 *
 * @param  string $insertion    	Likely HTML markup, ad script code etc.
 * @param  int    $paragraph_id 	After which paragraph should the insertion be added. Starts at 1.
 * @param  string $content      	Likely HTML markup.
 *
 * @return string               	Likely HTML markup.
 */
function prefix_insert_after_paragraph( $insertion, $paragraph_id, $content ) {
	$closing_p = '</p>';
	$paragraphs = explode( $closing_p, $content );
	foreach ($paragraphs as $index => $paragraph) {
		// Only add closing tag to non-empty paragraphs
		if ( trim( $paragraph ) ) {
			// Adding closing markup now, rather than at implode, means insertion
			// is outside of the paragraph markup, and not just inside of it.
			$paragraphs[$index] .= $closing_p;
		}

		// + 1 allows for considering the first paragraph as #1, not #0.
		if ( $paragraph_id == $index + 1 ) {
			$paragraphs[$index] .= $insertion;
		}
	}
	return implode( '', $paragraphs );
}