laravel config file retun null in config folder

I have a config file called fcm.php in config folder in laravel. The file looks like this:

<?php
return [
    'driver' => env('FCM_PROTOCOL', 'http'),
    'log_enabled' => true,

    'http' => [
        'server_key' => config('app.fcm_mode') == 'user' ? env('FCM_SERVER_KEY', 'Your FCM server key') : env('FCM_DELIVERY_SERVER_KEY', 'Your FCM server key'),
        'sender_id' => config('app.fcm_mode') == 'user' ? env('FCM_SENDER_ID', 'Your sender id') : env('FCM_DELIVERY_SENDER_ID', 'Your sender id'),
        'server_send_url' => 'http://ift.tt/1TmHkUo',
        'server_group_url' => 'http://ift.tt/1mkLeNy',
        'timeout' => 30.0, // in second
    ],
];

It works properly in my localhost(I vendor:publish fcm.php config file), But in a shared host, config('app') return null in this file(config/fcm.php)

What is the problem? Thanks

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