WordPress: pre_get_posts action combine date_query and meta_query

Question

In my WordPress project, I have gotten some problem.

I have many categories. The categories have a child category name New product.

Currently, I have used pre_get_posts to get the posts list in New product pages. If a post which have post_date > 2018/1/15 will be get in the category pages.

I want to add a custom checkbox keep_product in product which will keep product in New product page if post_date < 2018/1/15 when it checked.

So I think the conditions should be post_date > 2018/1/15** OR **keep_product = 1

I try to create a query, but it seem WP_Query only apply AND condition for date_query and meta_query.

function pre_get_posts_function($q){
  //Code in here
    $query->set('date_query', array(
        array(
           'after' => '2018/1/15'
        );
    ));

    $query->set('meta_query', array(
      array(
            'key' => 'keep_product',
            'value' => '1'
        )
    ));
}

I also try to create a posts_where action, but It ignores category condition. The product which is checked keep_after_60_days checkbox will be show in New product pages.

function pre_get_posts_function($q){
    //Code in here
     $query->set('date_query', array(
        array(
           'after' => '2018/1/15'
        );
    ));
    add_filter( 'posts_where', 'postmeta_new_releases',10,1);
}

function postmeta_new_releases( $where = '' ) {
    global $wpdb;
    $where .= "  OR ($wpdb->postmeta.meta_key = 'keep_product' AND $wpdb->postmeta.meta_value = 1)";   

    return $where; 
}

Could anyone help me. I get mad with it in some days.

Thanks!!