Welcome to the KPN forum @JakubWrz! In order for the tv-box to work, it needs an internet connection. You can achieve this in a number of ways, even if a direct line between the modem and tv-box isn't possible. 

The first option you could choose is the SuperWifi. It has a port for an internet cable. You can use this port to connect to the tv-box. It will also improve your wifi. If you need this in your house to have proper wifi coverage you may be eligible for a discount if you follow the steps on our Wifi Manager. 

The second option is a DVS. It is designed to transfer internet signal over powerlines. It does have the downside that other appliances may disrupt this signal. Things like certain types of lights or chargers. 

The caviat for both of these is that it does not fall under warranty. The equipment itself does. You have the standard consumer protections and return policies. But we can't guarantee that your tv signal will always work. If it does show problems with the signal you may need to provide a wired connection after all. If you can't do this youself we do have engineers who you can help with this. Terms and conditions are on our page about engineers.  

Hello ,

Thank you very much for your advice, I have found a solution but it will require pushing about 15 metres of Ethernet cable through the narrow pipe that runs from where the router is to the TV. I'm not sure I can manage this, I understand that I can count on your support and help from fellow service people? Greetings Jakub

@JakubWrz absolutely!  Other forumusers and me will help you digitally through here as best as possible. 

Will you let me know here when you succeeded? Or if there are more questions?