Add album gallery images to REST API

functions.php

add_action( 'rest_api_init', 'add_album_gallery_fields' );
function add_album_gallery_fields() {
    register_rest_field(
    'dt_gallery', 
    'album_gallery', 
    array(
        'get_callback'    => 'get_album_gallery', // custom function name 
        'update_callback' => null,
        'schema'          => null,
         )
    );
}


function get_album_gallery( $object, $field_name, $request ) {
    $mediaitems = get_post_meta( get_the_ID(), '_dt_album_media_items', true );
    return $mediaitems;
}

Please try this code in your child theme functions.php