Install pfSense, disconnect your pfSense WAN and LAN interfaces, boot, on startup completion from the console select option 2) Set interface(s) IP addresses and set the addresses you want. Then connect up your WAN and LAN interfaces.
Thereafter there are many things that can be readily changed from the WEB GUI.
I used smoothwall for a while but a few years ago changed to pfSense, mainly because of the much better support for wireless interfaces. I had a very brief look at a smoothwall web page recently and it seemed the software version hadn't changed since I last used it which would suggest the wireless support is even more behind pfSense.