If Apple Pay is not working on your iPhone then it is not an issue with the credit card.
Also, adding credit cards to Apple Pay is different than debit cards. With debit cards you can verify them in the wallet app through inputting a code that you receive via a text message (as well as other methods).
With a credit card, you can start the process of adding the card on your phone but you MUST ring Ulster Bank credit card services to enable the credit card in Apple Pay. Follow the on-screen instructions to do this.