Search on custom post type table - php

Question

I wanted to use the default search functionality using this :
The problem is I've made this table using this line of code
function waiting_list_column_content_callback($column_name, $post_ID) {
$guestID = get_post_meta( $post_ID, 'guest', true );
$user_info = get_userdata( $guestID );
$floorplanID = get_post_meta( $post_ID, 'floorplan', true );
$notes = get_post_meta( $post_ID, 'notes', true );
$waiting_list = wpr_get_waiting_list( $floorplanID );
$list_location = wpr_get_list_location( $post_ID, wpr_get_waiting_list( $floorplanID ) );
if ( $column_name == 'fname' ) {
echo ''.$user_info->first_name.'';
}elseif ( $column_name == 'lname' ) {
echo ''.$user_info->last_name.'';
}elseif( $column_name == 'floorplan' ){
echo get_the_title($floorplanID);
}
elseif( $column_name == 'logs_print' ){
echo 'Print';
}elseif( $column_name == 'list_location' ){
echo $list_location;
}
}
I've tried this and this. It work, but I can only search the user by "id".Since "id" is the only custom_field available to use.
This is the code I'm using to get results by id. "guest" is the "id" of the user. Which I use to display the last name and the first name of the user.
function extend_admin_search( $query ) {
//Extend search for document post type
$post_type = 'wpr_waiting_list';
// Custom fields to search for
$custom_fields = array(
"guest",
);
if( ! is_admin() )
return;
if ( $query->query['post_type'] != $post_type )
return;
$search_term = $query->query_vars['s'];
// Set to empty, otherwise it won't find anything
$query->query_vars['s'] = '';
if ( $search_term != '' ) {
$meta_query = array( 'relation' => 'OR' );
foreach( $custom_fields as $custom_field ) {
array_push( $meta_query, array(
'key' => $custom_field,
'value' => $search_term,
'compare' => 'LIKE'
));
}
$query->set( 'meta_query', $meta_query );
};
}
add_action( 'pre_get_posts', 'extend_admin_search' );
What I've wanted to do was search this custom post type by "first name" or "last name" .
I've tried some sql codes but I'm getting some "headers" error I don't understand.