Problem uploading base64 image (PNG) file to another server using FTP

Hello all, I'm hoping someone can help me with a problem I'm having when trying to upload an image (created from Signature Pad via base64) to another server using FTP. I'm using Laravel 7.0.

The code below works perfectly for storing a signature on my installation of Laravel, I get a nice PNG file in a year/month folder with the signature exactly how it was entered.

So the local part of the code works perfectly.

However, when I use the last line of code to upload the image, it does create a folder (called 'staff-signatures') on the remote server, but doesn't upload the file - it immediately throws an error: 'Failed to load resource: the server responded with a status of 401 (Unauthorized)'.

I've put my FTP details in the filesystems.php file as follows:

'ftp' => [
	'driver' => 'ftp',
	'host' => env('FTP_HOST'),
	'username' => env('FTP_USERNAME'),
	'password' => env('FTP_PASSWORD'),
	'root' => env('FTP_ROOT'),
	'passive' => false,
	'port' => 21,
],

All my FTP variables are in the .env file and they definitely work as I am able to make the folder on the other server if it doesn't exist.

I've never stored files off-server via FTP so it could be something really simple but I've been pulling my hair out trying to fix it but with no luck!

Hopefully the comments will help you through the code - does anyone have any ideas why this is happening?

Thanks in advance!

// Grab the signature
$signature = $request->sign;
$first_name = $request->first_name;
$last_name = $request->last_name;

// Set up our directory structure
$directory = 'staff-signatures';
$year = date('Y');
$month = date('m');
$localpath = $directory . '/' . $year . '/' . $month . '/';

// Check folder exists for storing
	if (!File::exists($directory . '/' . $year)) {
		File::makeDirectory($directory . '/' . $year, 0777, true, true);
	}
	if (!File::exists($directory . '/' . $year . '/' . $month)) {
		File::makeDirectory($directory . '/' . $year . '/' . $month, 0777, true, true);
	}

// Prepare signature
$image_parts = explode(";base64,", $request->sign);
$image_type_aux = explode("image/", $image_parts[0]);
$image_type = $image_type_aux[1];
$image_base64 = base64_decode($image_parts[1]);
$uniqueID = uniqid();
$filename = preg_replace(
	"/[^a-z0-9\.]/",
	"-",
	strtolower($first_name . '-' . $last_name . '-' . $uniqueID)
);

// Put it all together          
$file = $localpath . $filename . '.' . $image_type;

// Save the file to this laravel installation (works fine - I get my image)
file_put_contents($file, $image_base64);

//
// This doesn't work - I want to connect via FTP and store the above in the relevant folder
//
Storage::disk('ftp')->put('/staff-signatures/' . $filename . '.' . $image_type, $image_base64), 'r+');