Can't update IBAN in MijnKPN

Hi,

I signed up for KPN in February but it took a month to activate.

In the meantime, I’ve also closed my BUNQ bank account and am trying to replace it with my ING one, but it doesn’t update in MijnKPN (I confirm the switch, but when the page updates, it goes back to the old IBAN).

How can we make sure KPN has the right IBAN on file, can get paid, and doesn’t suspend my service next month? 

Thanks!