Go to the browser window with your Fritzbox settings. To get to the DHCP sever settings of your router, make sure that the view is set to “Advanced”.
You can find the DHCP server settings at Home > Home Network Overview > Network Settings > IPv4 Addresses. Remove the checkmark from the checkbox “Activate DHCP server” for the IPv4 settings of the Fritzbox.
Finally, click the button “OK” at the bottom of the page.