Jonathan
Mortgage company wants to collect the money from you. If you've agreed on payment by direct debit, they have to accept your NL IBAN according to EU law (regulation 260/2012).
You can basically tell them to fix problems on their side - it's not your problem. The question is, if and how much you're willing to stand up and fight for your rights. I would imagine that worst case, if things escalate, they could initiate debt collection or take you to court - though I can't imagine how you could lose there.
