solved it.
hopefully someone from wordpres.org helped me.
here is the link for anyone in the future faced the same problem as me.
wordpress.org page
the answer was:
If you want to add some user meta info in user database while registration you need to use “user_register” filter like below code:Add code in your themes function.php file
add_action( 'user_register', 'my_user_meta_save', 10, 1 );
function my_user_meta_save( $user_id ) {
if ( isset( $_POST['first_name'] ) )
update_user_meta($user_id, 'first_name', $_POST['first_name']);
}