If I understand it correctly, you have always paid monthy using iDeal? And now it doesn’t work anymore? Normally if you used iDeal for the first payment this will remain the standard payment type until you change it to something else.

Please check in your account what the payment type is. You can access it via “ Mijn abonnement beheren” and than “Payment type”. Check if you can see the iDeal logo. And maybe try to change the payment type.