Adopting Unifi Security Gateway
If you buy a Unifi Security gateway, out of the box the lan port is 192.168.1.1/24, which didn't work in my case as I wanted to configure the network interface on a different subnet.
It's fairly easy to do from the command line:
configure edit interfaces ethernet eth1 set address <new cidr>/24 delete address 192.168.1.1/24 set-inform http://ip-of-controller:8080/inform commit
Once everything comes up, check the unifi software to see if you can adopt the new gateway.
Once adopted, you can edit the all of the port info from within the unifi controller software.