If your customers are having failed payments and an “N:-5103:Cardholder did not return from ACS” error code from Tyl By Natwest’s transaction error logs, it is most likely caused by your customer being unable to confirm the payment for their bank.
You can only see this error when a customer is shown the 3D secure and then is asked by their bank to confirm the payment but the customer failed in providing the confirmation. Some banks need customers to confirm the payment and there is a time limit for which they must do this.
Due to the nature of taking payments online, 3D secure from time to time can occur from time to time. For this, we would recommend reaching out to the Tyl By Natwest support team and they should be able to sort out the issues for both your site and for your customer.