PHP how to download file send by X-Sendfile?

Question

I have a WordPress site A and send zip file with PHP-X-Sendfile, for example: https://a.com/?download=a will download file a.zip[

if (!empty($_GET['download'])) {
    $xSendFile = new PhpXsendfile();
    $xSendFile->download(WP_CONTENT_DIR . '/a.zip');
}

it's working perfectly.

and now I want to download it from WordPress site B. for example: https://b.com/?download=a can download file from https://a.com/?download=a.

I tried methods below:

Method 1:

$file_url = 'https://a.com/?download=a';
$a = download_url($file_url, 3000);
die;

Result: Not working, and return WP_Error A valid URL was not provided.

Note: download_url is a built-in WordPress function for download external file.

Method 2:

$file_url = 'https://a.com/?download=a';
$xSendFile = new PhpXsendfile();
$xSendFile->download($file_url);`

Result: Not working, and got browser error 'This site can’t be reached'.

So I think the problem is how to download remote file send by X-sendfile?

Any ideas? thank you.