Search across all network blogs in elasticpress

functions.php

<?php

/**
 * Search across all network blogs with elasticpress
 *
 * @param $scope string Search scope
 *
 * @return string
 */
function motivast_ep_search_scope( $scope ) {
    return 'all';
}

add_filter( 'ep_search_scope', 'motivast_ep_search_scope' );