It can take 3-4 business days from when the tenant registers for a Betalingsservice agreement via the provided link, until it becomes visible to you as a landlord in BoligPortal's rent collection system. It is simply a delay.
This is because the information needs to go through the bank, Betalingsservice, and finally BoligPortal. Therefore, it may be completely accurate if your tenant informs you that he/she has registered for Betalingsservice, even though you as the landlord cannot see it yet.
If after 3-4 days you still cannot see the registration on BoligPortal, despite the tenant having signed up, you are very welcome to contact BoligPortal's customer service.