OpenWRT on embedded device as firewall

I have the following setup in my network: My internet comes in through an ATT Uverse modem/router. The router is in bridge mode because that ATT Uverse modem/router connects to a second higher performance router I got from my company. That high performance router then connects to various internet network components (a switch, my TV, etc) as well as to a couple of Wireless Access Points so the house is filled with WIFI. What I want to do is to place an embedded device (like a Raspberry Pi or Banana Pi) running OpenWRT between my ATT Uverse Modem/Router and my high performance router and run intrusion detection/prevention software on this. In other words, internet signal comes in thru ATT Uverse modem/bridge router, goes through my OpenWRT embedded box, passes through to my high performance router and then into the rest of my network.

But I'm a little confused as to how to configure OpenWRT to make this happen. I found this link: http://ift.tt/1ffm9E0 but it looks like the "firewall" he is using actually ends up being a router. I don't want to eliminate my existing routers - I just want to place my embedded OpenWRT box between them. What do I have to do to make this happen?

If I place this box outside my router (in other words, between my router and modem) then is it the case that this box will no longer have an internal IP address? Must I instead place it behind my router to get an IP address?

Any help or suggestions would be appreciated.