The surface was a gift to welcome you as a customer, you'll need to return the modem and tv box if you have them. With all the hardware you also need to return the power adapters. 

When you cancel you'll receive an return box from kpn and you can send it back without cost at every postnl punt.

You can also send it back in your own box, in that case it is important to put in a paper with your client number in the box, the client number (klantnummer) is a number you can find on your monthly invoice or the MijnKPN app.

The return address is:

KPN

Antwoordnummer 10119

2400 VB Alphen aan den Rijn

Make sure to collect the shipping confirmation from the PostNL punt so you can prove that you have send it back.

Thank you!

Edited my last commend.