Show my custom post id if a country or ip

I believe you can do something like this (not tested):

if($_SERVER['REMOTE_ADDR']=="89.0.0.0" && $post->ID==15){

 $post5 = get_post('5');
 echo $post5->post_title;

 //Call custom meta using $post5->ID;

}

Update: If you’d like to replace the original object you can do the following:

if($_SERVER['REMOTE_ADDR']=="89.0.0.0" && $post->ID==15){
global $post;
$post = get_post('5');
setup_postdata($post);
//Here you can call for the_content() or the_title etc.
wp_reset_postdata(); //This function will bring back the old post id
}