How do I configure my Asus RT-AX59U to obtain IPV6?

Hello ​@Sina1961 , thanks for your message and welcome to our Community :)
To be honest, I have no experience with this type of device so I used the internet to find a way for you to obtain an IPv6 address

KPN uses IPv6 Dual Stack with DHCPv6. You should configure it like this:

1. Log in to your router at http://192.168..1.1 or via http://router.asus.com 

2. Go to Advanced Settings > IPv6.

3. Set Connection Type to Native.

4. Make sure DHCP-PD (Prefix Delegation) is enabled.

5. Keep Router Advertisement and DHCPv6 Server enabled for your LAN.

6. (Optional) Set custom IPv6 DNS servers like Google's:

   • 2001:4860:4860::8888

   • 2001:4860:4860::8844

7. Save the settings and reboot the router.

After rebooting, your router should receive an IPv6 address and a prefix from KPN. You can check if everything works via https://test-ipv6.com. Will you let us know if this was helpful for you?