I have had a discussion with their support desk.

They say it's a problem between your provider and the cloud services (Amazon) which is used.

Therefore I should contact my provider....

Part of the data should not come through to the cloud environment and that's why the list is not updated correctly.

Their advice was to clone your MAC adress in the Deco app so a 'new route / account' is created on the cloud and that should fix it.

For me it didn't...

I mentioned that's their product that is failing so they should contact the provider (Ziggo). Answer TP-Link: they are ignoring/ not acknowledging the problem so they don't help to investigate. 

Because of that I don't think they will start an investigation when I call them hahaha. I will write an review that this product isn't working properly for Ziggo (one of the biggest providers in the Netherlands) so users should look to a different brand..