Enabled Revisions to existing custom post type not working Wordpress

I am trying to enable revisions for an existing custom post type. As the post type was already created about 2 years ago so where the post type was register I added revisions to the supports array.

Earlier my code was like:

   $labels = array(
    'name' = 'Products',
    'singular_name' = 'Product',
    'all_items' = 'All Products',      
    'add_new' = 'Add New',
    'add_new_item' = 'Add New Product',
    'edit_item' = 'Edit Products',
    'new_item' = 'New Product',
    'view_item' = 'View Product',
    'search_items' = 'Search Products',
    'not_found' =  'No Prducsts Found',
    'not_found_in_trash' = 'No Products in Trash', 
    'parent_item_colon' = ''
  );
  $supports = array(
    'title',
    'custom-fields',
    'editor',
    'thumbnail'
  );
  $args = array(
    'labels' = $labels,
    'public' = true,
    'menu_position' = 5,
    'rewrite' = array( 'slug' = 'products' ),
    'has_archive' = true,
    'supports' = $supports 
  ); 
  register_post_type('products', $args);

And now it looks like:

   $labels = array(
    'name' = 'Products',
    'singular_name' = 'Product',
    'all_items' = 'All Products',      
    'add_new' = 'Add New',
    'add_new_item' = 'Add New Product',
    'edit_item' = 'Edit Products',
    'new_item' = 'New Product',
    'view_item' = 'View Product',
    'search_items' = 'Search Products',
    'not_found' =  'No Prducsts Found',
    'not_found_in_trash' = 'No Products in Trash', 
    'parent_item_colon' = ''
  );
  $supports = array(
    'title',
    'custom-fields',
    'editor',
    'thumbnail',
    'revisions'
  );
  $args = array(
    'labels' = $labels,
    'public' = true,
    'menu_position' = 5,
    'rewrite' = array( 'slug' = 'products' ),
    'has_archive' = true,
    'supports' = $supports 
  ); 
  register_post_type('products', $args);

This code didn't enable revisions in the custom post type.

Then I found another code which I added to my theme's function.php:

function add_revisions_custom_post()
{
    add_post_type_support( 'products', 'revisions' );
}
add_action('init','add_revisions_custom_post');

This one is also not working.

Can anyone suggest me how I can enable revisions?

Thanks in advance.

EDIT: Here is my wp-config.php file.

?php
/**
 * The base configurations of the WordPress.
 *
 * This file has the following configurations: MySQL settings, Table Prefix,
 * Secret Keys, WordPress Language, and ABSPATH. You can find more information
 * by visiting {@link http://codex.wordpress.org/Editing_wp-config.php Editing
 * wp-config.php} Codex page. You can get the MySQL settings from your web host.
 *
 * This file is used by the wp-config.php creation script during the
 * installation. You don't have to use the web site, you can just copy this file
 * to "wp-config.php" and fill in the values.
 *
 * @package WordPress
 */

// ** MySQL settings - You can get this info from your web host ** //
/** The name of the database for WordPress */
define('WP_MEMORY_LIMIT', '64MB');

define('DB_NAME', 'MYDBNAME');

/** MySQL database username */
define('DB_USER', 'MYDBUSER');

/** MySQL database password */
define('DB_PASSWORD', 'MYDBPASSWORD');

/** MySQL hostname */
define('DB_HOST', 'localhost');

/** Database Charset to use in creating database tables. */
define('DB_CHARSET', 'utf8');

/** The Database Collate type. Don't change this if in doubt. */
define('DB_COLLATE', '');

define('WP_MEMORY_LIMIT', '64MB');

/**#@+
 * Authentication Unique Keys and Salts.
 *
 * Change these to different unique phrases!
 * You can generate these using the {@link https://api.wordpress.org/secret-key/1.1/salt/ WordPress.org secret-key service}
 * You can change these at any point in time to invalidate all existing cookies. This will force all users to have to log in again.
 *
 * @since 2.6.0
 */
define('AUTH_KEY',         'c7ypbekexsfbt4wiucppwewene93cbtcfhf6xgpggepycabhildsoyqpre5iv3wi');
define('SECURE_AUTH_KEY',  'ujjxufvylpndsj0qeuwa90gxawj3cgaqnyusrdbmujmb08r37pnyreorpcyxqouu');
define('LOGGED_IN_KEY',    'vwlhlmenthgrq9g5jcocihz4ndldhrpegmcp6qyb3rfmjvxjejbacv1zharaexlp');
define('NONCE_KEY',        'qhnyqgmqckh3ylasveugagqlvifiuqajl6s9e7ulfrxepdxh2mewr8qhdinua8o2');
define('AUTH_SALT',        'rlb723gcatjvkfrd3jscmvdjio3kx9apm5yie9e4ibxktnnlukvgfpgbdsohrns9');
define('SECURE_AUTH_SALT', 'pllenyye8zbtml91hekptc2clqr7bhvhlriecz5qozexfhiqptmcvxrlehj44c16');
define('LOGGED_IN_SALT',   '9lyqc6qod0zdgyh6esp7bsxmpmuyp3h64m62pcnwxyefejh6tjykm7tpxhecg3xy');
define('NONCE_SALT',       'ob0bq3fye46rubbgu5flycjuai4ygxqgxho8bb1k8t81mwhghcbkysrgxrjzx0fu');

/**#@-*/

/**
 * WordPress Database Table prefix.
 *
 * You can have multiple installations in one database if you give each a unique
 * prefix. Only numbers, letters, and underscores please!
 */
$table_prefix = 'wp_';

/**
 * WordPress Localized Language, defaults to English.
 *
 * Change this to localize WordPress.  A corresponding MO file for the chosen
 * language must be installed to wp-content/languages. For example, install
 * de_DE.mo to wp-content/languages and set WPLANG to 'de_DE' to enable German
 * language support.
 */
define ('WPLANG', '');

/**
 * For developers: WordPress debugging mode.
 *
 * Change this to true to enable the display of notices during development.
 * It is strongly recommended that plugin and theme developers use WP_DEBUG
 * in their development environments.
 */
define('WP_DEBUG', false);

/* That's all, stop editing! Happy blogging. */

/** Absolute path to the WordPress directory. */
if ( !defined('ABSPATH') )
    define('ABSPATH', dirname(__FILE__) . '/');

/** Sets up WordPress vars and included files. */
require_once(ABSPATH . 'wp-settings.php');