You should be able to do something like this in your filter function:
function prefix_insert_post_ads( $content ) {
if ( is_admin() ) {
return $content;
}
if ( ! is_single() ) {
return $content;
}
if ( ! is_amp_endpoint() ) {
$ad_code = My Ad code;
return prefix_insert_after_paragraph( $ad_code, 4, $content );
}
// We must be on amp:
return amp_insert_after_paragraph( ... );
}
Then define your amp_isnert_after_paragraph
function to insert the newsletter where you want.