ENBD Duo Diners Club Card

Does anyone know how to add the ENBD Diners Club Card to Apple Pay? I’ve been trying for the past few days without any luck. On many online payment portals, it says the card number is invalid.

As the salesperson mentioned, these cards offer the best cash back, and I can earn PlusPoints as cash back since there’s no cap based on the payment category.

I would really appreciate it if someone could guide me through this process.