Ajax call within WordPress wont work at all (500 Internal Server Error)

WordPress way to make an Ajax call in a custom page template is unbelievable complicated. I cant get it to run at all without crashing my whole site. Why I need to do it that way? My other Ajax calls are not needing this complicated and complex way too.

So what I am trying to do:

I want to add a feature to the Shop that Resellers can enter there ID and reorder new bags for there Shops. So long so good.

I got my landing page:

    <?php
    /*
    Template Name: 
    */
    ?>
    <?php
        get_header();
    ?>
    <section id="main" class="reseller-page">

    <form action="<?php echo get_stylesheet_directory_uri(); ?>/reseller.php" method="GET" id="resellerform">
        Reseller ID: <br><input type="text" name="rid" id="rid">
        <button id="resellercheck">SUCHEN</button>
    </form> 
    <div id="resellerwrapper" style="display: none;">
        <div id="resellerinfo"></div>

        <div id="resellerorder" style="display: none;">
            <h2>Bestellformular:</h2>
            <?php
            $args = array('post_type' => 'product', 'posts_per_page' => -1, 'product_cat' => 'tabak-taschen');
            query_posts($args);
            if (have_posts()):
                echo '<div id="reseller-select-wrapper"><select id="artikelselector">';
                while (have_posts()) : the_post();
                    echo '<option value="' . $product->get_sku() . ': ' . $product->get_title() . '" data-type-href="' . get_permalink() . '">' . $product->get_sku() . ': ' . $product->get_title() . '</option>';
                endwhile;
                echo '</select>Menge: <input id="reseller-menge" type="text" class="input-text"><button value="hinzufügen" id="reseller-add-to-form">+</button></div>';
            endif;
            ?>
            <form action="" method="POST" id="reseller-bestell-liste">
                <ul>

                </ul>
                <textarea name="nachricht" id="reseller-msg"></textarea>
                <input type="submit" id="reseller-send-order" value="bestellen" title="verbindliche Bestellung" />
            </form> 

        </div>
    </div>
</section>
<?php
get_footer();
?>

then the checkresellerid.php that checks the database if the RiD(ResellerID) is in the DB:

<?php
//header('Content-Type: text/html; charset=utf-8');
include 'db_connect.php';

// Check connection
if (mysqli_connect_errno()) {
echo "Failed to connect to MySQL: " . mysqli_connect_error();
}

if (isset($_GET['rid'])) {
//  $rid = "R10369";
$rid = $_GET['rid'];


$result = mysqli_query($con, "SELECT rid FROM reseller WHERE rid = '" . $rid . "'");

$ergebnis = mysqli_num_rows($result);

if ($ergebnis == 1) {
    $RJSON = [];
    $RQUERY = mysqli_query($con, "SELECT * FROM reseller WHERE rid = '" . $rid . "'");
    while ($row = mysqli_fetch_array($RQUERY, MYSQL_ASSOC)) {

        $RJSON[] = array(
//          'rid' => $row['rid'],
            'name' => $row['name'],
            'adresse' => $row['adresse'],
            'email' => $row['email'],
            'phone' => $row['phone']

        );
    }


    echo json_encode($RJSON);

} else {
    echo 'false';
}

mysqli_close($con);
}
?>

When I enter something in the Input Field(no matter if its in the DB or not) I get a 500 Internal Server Error. But why my other Ajax calls on the Site like my “Love a product” feature in the woocommerce shop loop are working but not this little fella? on my localhost is everything runnning correctly. Did it has anything to do because its a custom php file(template page file)?

Check the Example Page >>> Here <<<

I tried registering a function in functions.php but that crashed the whole site.
How can I pack my php script in this functions.php and call it correctly? Why is it even neccesary to make it that complicated? I am googling this for hours and tryied many ways, nothing works…

Thanks for any help and sorry for my bad englisch.

EDIT:

my functions.php looks like this:

<?php

/****************************************
Theme Setup
*****************************************/

require_once( get_template_directory() . '/lib/init.php' );
require_once( get_template_directory() . '/lib/theme-helpers.php' );
require_once( get_template_directory() . '/lib/theme-functions.php' );
require_once( get_template_directory() . '/lib/theme-comments.php' );


/****************************************
Require Plugins
*****************************************/

require_once( get_template_directory() . '/lib/class-tgm-plugin-activation.php' );
require_once( get_template_directory() . '/lib/theme-require-plugins.php' );

add_action( 'tgmpa_register', 'mb_register_required_plugins' );
add_theme_support( 'woocommerce' );


/****************************************
Misc Theme Functions
*****************************************/

/**
 * Define custom post type capabilities for use with Members
 */
function mb_add_post_type_caps() {
    // mb_add_capabilities( 'portfolio' );
}

/**
 * Filter Yoast SEO Metabox Priority
 */
add_filter( 'wpseo_metabox_prio', 'mb_filter_yoast_seo_metabox' );
function mb_filter_yoast_seo_metabox() {
    return 'low';
}


function checkResellerId(){
    include 'db_connect.php';
    wp_localize_script( 'function', 'checkResellerId', array( 'ajaxurl' => admin_url( 'admin-ajax.php' ) ) );

    // Check connection
    if (mysqli_connect_errno()) {
        echo "Failed to connect to MySQL: " . mysqli_connect_error();
    }

    if (isset($_GET['rid'])) {
    //  $rid = "R10369";
        $rid = $_GET['rid'];


        $result = mysqli_query($con, "SELECT rid FROM reseller WHERE rid = '" . $rid . "'");

        $ergebnis = mysqli_num_rows($result);

        if ($ergebnis == 1) {
            $RJSON = [];
            $RQUERY = mysqli_query($con, "SELECT * FROM reseller WHERE rid = '" . $rid . "'");
            while ($row = mysqli_fetch_array($RQUERY, MYSQL_ASSOC)) {

                $RJSON[] = array(
    //              'rid' => $row['rid'],
                    'name' => $row['name'],
                    'adresse' => $row['adresse'],
                    'email' => $row['email'],
                    'phone' => $row['phone']

                );
            }


            echo json_encode($RJSON);

        } else {
            echo 'false';
        }

        mysqli_close($con);
    }
}

add_action( 'wp_ajax_nopriv_product_s', 'checkResellerId' );
add_action( 'wp_ajax_product_s', 'checkResellerId' );

function resellerMail(){
    wp_localize_script( 'function', 'resellerMail', array( 'ajaxurl' => admin_url( 'admin-ajax.php' ) ) );
    $zieladresse = 'xxx';

    $absenderadresse = 'xxxReseller';

    $absendername = 'xxxReseller';

    $betreff = 'Reseller-Bestellung';

    $trenner = ":t";

    if ($_SERVER['REQUEST_METHOD'] === "POST") {

            $header = array();
            $header[] = "From: ".mb_encode_mimeheader($absendername, "ISO-8859-15", "Q")." <".$absenderadresse.">";
            $header[] = "MIME-Version: 1.0";
            $header[] = "Content-type: text/plain; charset=ISO-8859-15";
            $header[] = "Content-transfer-encoding: 8bit";

        $mailtext = "";

        foreach ($_POST as $name => $wert) {
            if (is_array($wert)) {
                        foreach ($wert as $einzelwert) {
                                $mailtext .= $name.$trenner.$einzelwert."n";
                }
            } else {
                $mailtext .= $name.$trenner.$wert."n";
            }
        }

        mail(
            $zieladresse, 
            mb_encode_mimeheader($betreff, "ISO-8859-15", "Q"), 
            $mailtext,
            implode("n", $header)
        ) or die("Die Mail konnte nicht versendet werden.");
        echo $mailtext; // nur check für ajax response, später wieder entfernen!!!!
        echo 'Ihre Bestellung wurde an uns versandt.<br>Sie werden telefonisch von uns kontaktiert um die Bestellung zu bestätigen.';
        exit;
    }

//header("Content-type: text/html; charset=utf-8");
}

add_action( 'wp_ajax_nopriv_product_s', 'resellerMail' );
add_action( 'wp_ajax_product_s', 'resellerMail' );

Screen of the 500 Error:
![Screenshot 500 Internal error][2]