Ajax based range search in jquery plugin "Datatables"

I´m using the jquery based table plugin "datatables" and I´m trying to implement an ajax based "range search" between two numbers ("start-date" and "end_date"). These entered values should be used for a query in the MySQL column "order_id".

On the server-sided script (fetch.php) I catch the both values like that.

if(isset($_POST['start_date'], $_POST['end_date'])) {
         $query .= 'order_id BETWEEN "'.$_POST["start_date"].'" AND "'.$_POST["end_date"].'" AND ';
 }

The problem is I can´t see any errors in the console, but after using the number range search no results are displayed.

The "category select menus" (category and category2) are working as expected.

I´ve setted up a test site, maybe you can help me to find the error: Testsite

This is my script:

$(document).ready(function () {
    var category = "";
    var category2 = "";
    var start_date = "";
    var end_date = "";

    load_data();

    function load_data(is_category, is_category2, start_date, end_date) {
        console.log(is_category, is_category2, start_date, end_date);
        var dataTable = $('#product_data').DataTable({
            "processing": true,
            "serverSide": true,
            "order": [],
            "ajax": {
                url: "fetch.php",
                type: "POST",
                data: {
                    is_category: is_category,
                    is_category2: is_category2,
                    start_date: start_date,
                    end_date: end_date
                },
            }
        });
    }

    // Number Range Search

    $('#search').click(function () {
        console.log($(this).attr('id'), start_date, end_date)
        var start_date = $('#start_date').val();
        var end_date = $('#end_date').val();
        if (start_date != '' && end_date != '') {
            $('#product_data').DataTable().destroy();
            load_data('','',start_date, end_date);
        }
        else {
            alert("Both Date is Required");
        }
    });

    // Select Menu id="category"

    $(document).on('change', '#category, #category2', function () {
        //console.log($(this).attr('id'), category, category2)
        if ($(this).attr('id') === "category") {
            category = $(this).val();
        } else if ($(this).attr('id') === "category2") {
            category2 = $(this).val();
        }
        // 
        $('#product_data').DataTable().destroy();
        if (category != '') {
            load_data(category, category2);
        }
        else {
            load_data();
        }
    });

    // Select Menu id="category2"

    $(document).on('change', '#category2', function () {
        var category2 = $(this).val();
        $('#product_data').DataTable().destroy();
        if (category2 != '') {
            load_data(category, category2);
        }
        else {
            load_data();
        }
    });

});

fetch.php

//fetch.php
$connect = mysqli_connect("localhost", "xxxxx", "xxxxx", "xxxxx");
$columns = array('order_id', 'order_customer_name', 'order_item', 'order_value', 'order_date');

$query = "SELECT * FROM tbl_order WHERE ";

if(isset($_POST['start_date'], $_POST['end_date']))
{
 $query .= 'order_id BETWEEN "'.$_POST["start_date"].'" AND "'.$_POST["end_date"].'" AND ';
}


if(isset($_POST["is_category"]))
{
 $query .= "order_item = '".$_POST["is_category"]."' OR ";
}

if(isset($_POST["is_category2"]))
{
 $query .= "order_customer_name = '".$_POST["is_category2"]."' AND ";
}

if(isset($_POST["search"]["value"]))
{
 $query .= '
  (order_id LIKE "%'.$_POST["search"]["value"].'%" 
  OR order_customer_name LIKE "%'.$_POST["search"]["value"].'%" 
  OR order_item LIKE "%'.$_POST["search"]["value"].'%" 
  OR order_value LIKE "%'.$_POST["search"]["value"].'%")
 ';
}

if(isset($_POST["order"]))
{
 $query .= 'ORDER BY '.$columns[$_POST['order']['0']['column']].' '.$_POST['order']['0']['dir'].' 
 ';
}
else
{
 $query .= 'ORDER BY order_id DESC ';
}

$query1 = '';

if($_POST["length"] != -1)
{
 $query1 = 'LIMIT ' . $_POST['start'] . ', ' . $_POST['length'];
}

$number_filter_row = mysqli_num_rows(mysqli_query($connect, $query));

$result = mysqli_query($connect, $query . $query1);

$data = array();

while($row = mysqli_fetch_array($result))
{
 $sub_array = array();
 $sub_array[] = $row["order_id"];
 $sub_array[] = $row["order_customer_name"];
 $sub_array[] = $row["order_item"];
 $sub_array[] = $row["order_value"];
 $sub_array[] = $row["order_date"];
 $data[] = $sub_array;
}

function get_all_data($connect)
{
 $query = "SELECT * FROM tbl_order";
 $result = mysqli_query($connect, $query);
 return mysqli_num_rows($result);
}

$output = array(
 "draw"    => intval($_POST["draw"]),
 "recordsTotal"  =>  get_all_data($connect),
 "recordsFiltered" => $number_filter_row,
 "data"    => $data
);

echo json_encode($output);