Hello @dharm 

You can't, the KPN modems do not support bridge mode.

If double NAT is not a problem then you can set your Wifi router as DMZ device on the KPN router, then all traffic will be forwarded to your Wifi router.

To do that you'll have to make a DHCP binding for your Wifi router and then set it as DMZ device in de KPN modem.

Another possibility is to set your Wifi router in AP mode (if it supports that) so it won't function as a router anymore.