Router - TG582N - Subnet Routing

This assumes you have a have 3 sets off addresses:

1. A /29 or /28 subnet that you want to route to your firewall server, lets say 80.2.2.0/28
2. A /30 subnet used for the router and firewall server, lets call them 90.1.1.0/30, the router has 90.1.1.1 and your firewall has 90.1.1.2
3. A single Ip address for the router ADSL link (allocated by DHCP)

First set the router up as normal on the GUI, selecting ADSL expert and on subnet type the independent router IP. This should deal with your /30 and single IP now working. I personally set the firewall to disabled and turned off all services including the wifi.

Telnet/ssh to your router

You need to do 2 things, add routing for the subnet to your firewall and add a mapping so that incoming connections work. (obviously substitute your own addresses here)

ip rtadd dst=80.2.2.0/28 gateway=90.1.1.2 static=enabled
nat mapadd intf=Internet type=nat outside_addr=80.2.2.0/28 inside_addr=80.2.2.0/28