Avoiding the 404 when pagination of a cpt or term of cpt does is under the posts per page settings in reading settings.

for-functions.php

<?php
//don't add



// posts per page for CPT 'your-cpt'
function yourprefix_posts_per_page_in_portfolio_cpt( $query ) {

	if ( ! is_admin() && is_post_type_archive( 'your-cpt' ) ) {
        $query->set( 'posts_per_page', 2 );
	}

}
add_filter( 'parse_query', 'yourprefix_posts_per_page_in_portfolio_cpt' );


// posts per page for term (your-term-slug) of CPT 'your-cpt'
function yourprefix_posts_per_page_in_term_of_portfolio_cpt( $query ) {

	if ( ! is_admin() && term_exists( 'your-term-slug' ) && ! is_post_type_archive( 'your-cpt' ) ) {
        $query->set( 'posts_per_page', 1 );
	}

}
add_filter( 'parse_query', 'yourprefix_posts_per_page_in_term_of_portfolio_cpt' );