Configure IPN/Callback URL/Function to intigtate EasyPay to Opencart v. 3.0.2.0

I am using EasyPay payment gateway on my Opencart store v. 3.0.2.0, the easypay has provided me an opencart extension and two pdf files for guideline to configure extension as well as IPN/Callback URL/Function, I could update the extension related information correctly and the payment gateway is processing payments correctly but I could configure IPN/Callback URL/Function properly therefore when customer make successful payment he is landing on the failure page of my store instead of success page. I am attaching both the guide files for configuring IPN/Callback URL/Function, please look to the relevant section and only submit bid if you are sure you can do it correctly. Many Thanks