Hello @Arend.ayl, welcome.

I understand you have trouble logging into the KPN Box 12. 

1. Open the browser and go to 192.168.2.254 or mijnmodem.kpn.
2. The username is ‘admin’. You can find the password on the sticker on the back of the KPN Box 12. 
3. After you log in, change your password to a new one.

Have you forgotten your password? Reset your router to get the password back to the password on the sticker.

Your browser might give you a security warning. You can click ‘Advanced/Geavanceerd’ to accept these risk without any problems. It's safe because you're staying within your own network. After this, you'll be redirected to http://mijnmodem.kpn.

Do you keep getting the security warning? Or does it keep saying the password is wrong? Then please, check the following:

• Check the password. Some characters look like each other, for example the letter o and the number 0. Also, if you have a ^ in your password, then don't forget to press 'space] before continuing. 
• Check if you're using http://, without the s.
• Delete the cookies en empty the cache.
• Don't use a VPN.
• Use the KPN DNS.

Still no success? Then try a different browser or try http://192.168.2.254/2.0/gui/#/login.

no afcourse it did not work i have tried everything like i said every method you can find my admin password goes like this ***

Admin: deleted password for privacy reasons

Oke, have you also tried connecting a laptop or PC directly to the modem using an ethernetcable? That way, you can change the password yourself.