Empty cart in ajax

Good afternoon, the problem I have today is that I need to modify the shopping cart from the checkout page.
I need that when clicking on specific buttons, an ajax request is sent to a server action that removes a product from the cart and adds it again with a different variation_id.
The checkout window should be updated automatically
The problem I have is that when I connect via ajax to the server the WC()->cart variable contains an empty array even though there are products in the cart (when I see this variable without ajax it returns the cart correctly)

Javascript:

function uac_change_cart(element) {
	//Cambiar carrito.
    var request = new XMLHttpRequest();
    request.open('POST', 'https://mywebsite.com/wp-admin/admin-ajax.php', true);
    request.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded;');
    request.onload = function () {
        if (this.status >= 200 && this.status < 400) {
            uac_update_cart_ui();
        } else {
            alert("Ha ocurrido un error actualizando su carrito. Por favor, vuelva a intentarlo");
        }
    };
    request.onerror = function () {
       	console.log("Error de conexión con el servidor. Compruebe su conexión a Internet");
    };

    request.send('action=uac_workspace_change_cart&payment=' + document.getElementById('uac_workspace_payment_mode').value + '&value=' + document.getElementById('uac_workspace_is_domain_purchase').checked + '&user_id=' + document.getElementById('uac_user_id').value);
}

PHP:

function uac_workspace_change_cart() {
    global $wpdb, $woocommerce;
    
    if (!isset($_POST["payment"]) || !isset($_POST["value"]) || !isset($_POST["user_id"]) || !isset($_POST["cookie"])) {
    	wp_send_json_error(402);
    }
    $workspace_product = "";
    $payment = $_POST["payment"];
    $value = $_POST["value"];
    $user_id = $_POST["user_id"];
    
    //this foreach is never executed because it detects the empty cart (When it isn't). The cases "XXXXX" are the id_product of the products i want to run this for.
    foreach (WC()->cart->get_cart_contents() as $cart_item) {
		$product_id = $cart_item["product_id"];
    	switch($product_id) {
        	case 25754:
            case 23628:
            case 23836:
            case 21909:
            case 15479:
            case 21903:
            case 21895:
            case 16943:
            case 15467:
            case 19535:
            case 21895:
            case 21893:
            case 16926:
            case 16931:
            case 15453:
            	$workspace_product = $cart_item;
                break;
        }
    }
    
    //If dont find the correct product in the cart give 404 to the client.
    if ($workspace_product == "") {
    	wp_send_json_error(404);
    }
	
    //Remove old item from cart:
    WC()->cart->remove_cart_item( $workspace_product["key"] );
    
    //Find the new variation_id (the product_id is the same).
    $variation_id = null;
    if ($payment == "YEARLY") {
    	$period = "ANUAL";
    } else {
    	$period = "MENSUAL";
    }
    
    if ($value == "true") {
    	$variation = "Quiero comprar un dominio";
    } else {
        $variation = "Ya tengo dominio profesional";
    }
    
    $sql = "SELECT id FROM ".$wpdb->prefix."posts WHERE post_parent=".$workspace_product["product_id"]." and post_excerpt='Facturación: ".$period.", Dominio profesional: ".$variation."'";
    
    $variation_id = $wpdb->get_results($sql, ARRAY_A)[0]["id"];
    //Now i have the new variation_id for insert in the cart.
    
    //Add the same product_id but different variation_id to the cart.
    WC()->cart->add_to_cart( $workspace_product["product_id"], $workspace_product["quantity"], $variation_id, null, null );
    
    //Trying to save the cart in session (dont work propertly i think).
    WC()->cart->set_session();
        
    wp_send_json_success(200);
}

add_action('wp_ajax_uac_workspace_change_cart','uac_workspace_change_cart');
add_action('wp_ajax_nopriv_uac_workspace_change_cart','uac_workspace_change_cart');

I guess the problem is that when connecting asynchronously (ajax) the woocommerce init doesn’t run or the session cart doesn’t load. How could I fix it?