Hi @Techrpr,
The most important question: are you a KPN customer? Or do you have a subscription with another provider? If you're not a KPN customer, you can't use a KPN Experia Box.
The previous owner of the house should have taken the Experia Box with him to the new address, or he should have returned it to KPN if he ended the contract when moving out.
If you order a KPN subscription, you will receive a KPN Experia Box, which is our modem/router. This Experia Box will remain KPN's property, if it breaks down or shows errors you will get a new one for free. As @AriënC already mentioned; you're allowed to switch to a router of your choice, if you prefer!