Custom post type by ID in root folder

i have a custom post type project

the url that worked out of the box was:

www.myapp.com/project/POST-ID

and

www.myapp.com/project/POST-SLUG

now my client needs the url be available in the root address. So I added the following code to functions.php

function na_remove_slug( $post_link, $post, $leavename ) {

  if ( 'project' != $post->post_type || 'publish' != $post->post_status ) {
    return $post_link;
  }

  $post_link = str_replace( "https://wordpress.stackexchange.com/" . $post->post_type . "https://wordpress.stackexchange.com/", "https://wordpress.stackexchange.com/", $post_link );

  return $post_link;
}
add_filter( 'post_type_link', 'na_remove_slug', 10, 3 );


function na_parse_request( $query ) {


  if ( ! $query->is_main_query() || 2 != count( $query->query ) || ! isset( $query->query['page'] ) ) {
    return;
  }

  if ( ! empty( $query->query['name'] ) ) {
    $query->set( 'post_type', array( 'post', 'project', 'page' ) );
  }
}
add_action( 'pre_get_posts', 'na_parse_request' );

this works now:

www.myapp.com/POST-SLUG

this does not work though:

www.myapp.com/POST-ID

is there any way I can make it work?

Try creating a custom rewrite rule and a custom rewrite tag. This should allow you to set up a regular expression to capture the ID and manipulate the query.

Rewrite API/add rewrite rule « WordPress Codex
https://goo.gl/kdEJd8
Rewrite API/add rewrite tag « WordPress Codex
https://goo.gl/PVLzLh