Having Trouble with Push Notifications to iOS devices with FCM

Question

I'm working on an app and have gotten FCM to send to notifications to Android devices, but currently having issues with sending to iOS devices.

I have followed the steps to do this as listed here, which also includes the steps listed here.

Here is the FCM SDK code that I have that works with Android devices:

    // This assumes that you have placed the Firebase credentials in the same directory
    // as this PHP file.
    $serviceAccount = ServiceAccount::fromJsonFile(__DIR__.'/google-service-account.json');

    // get our Firebase account loaded
    $firebase = (new Firebase\Factory())
            ->withServiceAccount($serviceAccount)
        ->create();

    // create a message
    $messaging = $firebase->getMessaging();

    // Creating a notification
    $notification = Notification::fromArray([
        'title' => $title,
        'body' => $body,
    ]);

    $data = [
        'click_action' => 'FLUTTER_NOTIFICATION_CLICK',
        'fromid' => $fromid,
    ];

    // Compose the message; notification + device token
    $message = CloudMessage::withTarget('token', $deviceToken)
        ->withNotification($notification)
        ->withData($data);


    // validate the message
    try {
        $messaging->validate($message);
    } catch (InvalidMessage $e){
        print_r($e->errors());
        exit;
    }

    // send message
    $report = $messaging->send($message);

I have tried sending messages through the Firebase console and using the FCM SDK; neither of which have caused a notification to appear on iOS devices. The console gives me no feedback, but the SDK gives me an error:

Fatal error: Uncaught GuzzleHttp\Exception\ClientException: Client error: POST https://fcm.googleapis.com/v1/projects/apwt-app1/messages:send resulted in a 401 Unauthorized response: { "error": { "code": 401, "message": "Auth error from APNS or Web Push Service", "status": "UNAUTHENTICATE (truncated...) in /var/www/vhosts/globalsups/composer/vendor/guzzlehttp/guzzle/src/Exception/RequestException.php:113