OS X 10.5.6 Server, DHCP renewal problem

[iguana9999]

I’ve got a Mini setup as a DHCP, NAT and firewall server for my internal network. Im using an USB -> Ethernet for my second Ethernet connection to the modem.
Setup is:

Modem (speedtouch) -> Mini connected thru the USB -> ethernet adapter (en2)
Mini -> Airport extreme N thru the built in Gigabit ethernet port (en0)

Due to problems with the modem, I was force to setup the modem to forward the external IP it gets from my ISP to my server. (i cannot forward ports but only put the miniserver in the DMZ of the speedtouch modem) This gives the Mini server the external IP of the ISP.

Now all works fine until my ISP renews the DHCP lease it gives out to the speedtouch modem. Internet connection on the clients will stop and en2 is stuck on the old IP it got from the modem. Manually renewing the IP on en2 on the server is stuck until I switch off the firewall. After the firewall is switched off the server gets a new IP from my modem (new ip from ISP) then my internet connection is restored and the firewall can be turned on again.
This cycle happens every 24hrs or so and i need a solution. I googled and found an article stating that if I add the router address from my ISP to my firewall the DHCP lease renewal would work. I tried this but to no avail.

Anybody got an idea how to setup the firewall or how to automate the shutdown of the Firewall and renewal of the DHCP lease?
Already tried ipconfig set en2 BOOTP and then back to DHCP, when using this i cannot get an IP even when the firewall is turned off and I’m forced to reboot the server.
After reboot all is well again.

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