I have followed the steps to complete the Return URL and IPN in PayPal Standard Sandbox for nopCommerce 3.10 but when PayPal returns to my website it returns to the home page and not the cart/completed page as I was expecting.
If I'm not mistaken, the CancelOrder method of the PayPal controller affects on where to return the user after clicking "Cancel" button on the PayPal site
After clicking the 'Cancel' button, the user is returned to the store and shopping cart is empty, so he can't return to the checkout. In this case, the user must go to the order details page, where he can re-checkout the order.
You can try to automatically redirect the user to the order detail page, using
return RedirectToRoute("OrderDetails", new { orderId = orderId });