how can i configur it as a gateway

i have an old microsoft isa 2004 need it just for spread internet not to work as fire wall i need thebroadband ton work the same. i gave it static ip and put it on line and configured 3 broadband...

model : TL-ER5120 ver : 2.0 firm ver : 108 Build 20140212 rel 36037

Model : TL-ER5120 Hardware Version : 2.0 Firmware Version : 108 Build 20140212 Rel 36037 ISP : At my work we have a domain server and i need to configure TL-ER5120 to work as firewall instead of our...