The additional fee for the COD payment method not visible in the front-end

Question

I'm trying to add an additional fee to the COD payment method.

The below code is working fine but only when I'm logetin as a administrator.

If I logout the fee doesn't show in the front end and I don't understand why.

For some reason the code is working only if I'm logget in as a administrator.

// add addition fee to cod payment method
if ( ICL_LANGUAGE_CODE=='cs' ){
    add_action( 'woocommerce_cart_calculate_fees','cod_fee' );
    function cod_fee() {
        global $woocommerce;

        if ( is_admin() && ! defined( 'DOING_AJAX' ) )
            return;
            // get your payment method
            $chosen_gateway = WC()->session->chosen_payment_method;
            //echo $chosen_gateway;
            $fee = 17;
            if ( $chosen_gateway == 'cod' ) { //test with cash on delivery method
            WC()->cart->add_fee( 'Delivery fee', $fee, false, '' );
        }
    }
}

// jQuery - Update checkout on payment method change
add_action( 'woocommerce_checkout_init', 'payment_methods_refresh_checkout' );
function payment_methods_refresh_checkout() {
    wc_enqueue_js( "jQuery( function($){
        $('form.checkout').on('change', 'input[name=payment_method]', function(){
            $(document.body).trigger('update_checkout');
        });
    });");
}