The Wifi light on the KPN Box 12 is not on

Have you tried pushing the button with the WiFi-symbol on the left-hand side of the device, @Yuliana.astro?

Yes, I did that but no effect. What else should I do? Thank you.

Have you perhaps accidentally  marked this as the best answer? Because it obviously is not…

It's really strange that the WiFi isn't working as it is generated by the device itself....perhaps it's best to call 0800-0402 (for free) as they can probably detect what's not functioning on your device, @Yuliana.astro.

@Ratonesque ops, I guess I accidentally marked that. I just had 30 minutes conversation with KPN staff and she guided me step by step to restart the modem/router (which I have done more than 3 times today). Unfortunately, the problem is still here. 
She is going to send me a new modem/router and we will see again after that. 

Yes, you probably need a new modem. I understand the WiFi worked well until you restarted the modem...obviously something went wrong here. Hopefully the new modem will provide you with a good WiFi-signal again.