cmb2 and wp-json

Hi, I’m trying to get cmb2 working in wordpress rest api, but unfortunately without luck.
I have enabled the display in the rest api as indicated in the documentation:

$opzioni_pagina = new_cmb2_box( array(
'id' => $prefix . 'metabox',
'title' => __( 'Opzioni', 'cmb2' ),
'object_types' => array( 'servizi' ),
'context' => 'normal',
'priority' => 'high',
'show_names' => true,
'show_in_rest' => WP_REST_Server::ALLMETHODS,
) ); 
$opzioni_pagina->add_field( array(
    'name' => 'Sottotitolo',
    'id'   => $prefix . 'sottotitolo',
    'type'    => 'text',
      'sanitization_cb' => 'prefix_sanitize_text_callback',
    'show_in_rest' => WP_REST_Server::ALLMETHODS,
) );

but when i go to call the url in get https://site.ext/wp-json/cmb2/v1/boxes, i get a 404:

{“code”:”rest_no_route”,”message”:”Nessun percorso fornisce una corrispondenza tra l’URL ed il metodo richiesto.”,”data”:{“status”:404}}

what am I doing wrong?