Modify the checkout page to show Register Helper fields after Billing Address fields.

pmpro-customizations.php

<?php

/**
 * This recipe modifies the checkout page to show Register Helper fields after Billing Address fields.
 * 
 * It is intended for use with the Register Helper Add On:
 * https://www.paidmembershipspro.com/add-ons/pmpro-register-helper-add-checkout-and-profile-fields/
 * 
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

function my_pmprorh_init() {
	remove_action( 'pmpro_checkout_boxes', 'pmprorh_pmpro_checkout_boxes' );
}
add_action( 'init', 'my_pmprorh_init' );

function mypmpro_checkout_after_billing_fields() {
	add_action( 'pmpro_checkout_boxes', 'pmprorh_pmpro_checkout_boxes' );
	do_action( 'pmpro_checkout_boxes' );
}
add_action( 'pmpro_checkout_after_billing_fields', 'mypmpro_checkout_after_billing_fields' );