I tried your approach, it is working for me if i enter the quantity in quantity textfield. But it doesn't work if i use increment/decrement signs to update the quanity. Can you please guide me how can do this. 

    $('.qty-down-fixed-onclick-page-cart, .qty-up-fixed-onclick-page-cart').on('click',  ajaxQuantityUpdate);

Above code works but only once. After ajax call it stops working.