Redirects members-only content to the Membership Levels page if a user is logged out or not a member in a multisite network using the PMPro Multisite Membership Add On.

my_template_redirect_require_membership_access.php

<?php
/*
 * Redirects members-only content to the Membership Levels page if a user is logged out or not a member.
 */

function my_template_redirect_require_membership_access() {
	if ( function_exists( 'pmpro_has_membership_access' ) && ! pmpro_has_membership_access() ) {
		if ( ! is_multisite() && ! function_exists( 'pmpro_multisite_membership_init' ) ) {
			wp_safe_redirect( pmpro_url( 'levels' ) );
			exit;
		} else {
			wp_safe_redirect( get_blog_permalink( get_current_site()->id, get_blog_option( get_current_site()->id, 'pmpro_levels_page_id' ) ) );
			exit;
		}
	}
}
add_action( 'template_redirect', 'my_template_redirect_require_membership_access' );