Is there any reason to leave your current router in place? If you have to because it is being used as a modem, does it have a bridge/modem only mode, so it passes its external IP through to IPF?
Alternatively, reconfigure your IP router to a different LAN IP/subnet, e.g 192.168.30.2/24 and then configure IPF with its LAN on 192.168.60.2/24 and a WAN on 192.168.30.x. You may need to configure IPF offline. Having reconfigured both, you should then be able to unplug all your LAN from the IP router and plug it into IPF and plug the IPF WAN into your IP router.