Hi @FrancescBox, welcome to our forum!
The easiest way would be to just return it in a box with your customer number or adress on it.
You can use our free return-adress:

Thanks @Thomas , can I return it before getting all the papers required, like the gemeente exit ones? Because, as I said, by the time I get them I will be already abroad.

Yes no problem, you can return them whenever you want really!

I canceled my account, and did not get yet the folded box to send the modem back to KPN. Who should I contact ?

Hello @Adva  see this website from KPN:

https://www.kpn.com/service/administratie/terugsturen.htm#/ser-terugsturen/ser-terugsturen-huur-bruikleen-opzeggen-annuleren

You can use your any empty box you can get, from the supermarket, by example.

Good luck.

...

@Adva 

When using a box of your own don't forget to insert a note with your customernumber (klantnummer) and name and address.