Payment Page on seperate domain not taking visitor to thank you page after payment made

Website Platform - WordPress
Payment Gateway - CashFlow

Issue: When a visitor places an order, the payment page loads a hosted payment form on gateway.cashflows/payment. Once payment is complete, it should take the user back to /thank-you. It does this correctly when using the website directly; however, the app takes the user back to /checkout/ even after payment has been made successfully.

This results in duplicate payments as it is unclear whether payment has been made.

Do you have any guidance on how to resolve this?

Thanks