Changing USG-3P WAN static IP aka “There was an error saving the Primary (WAN1) network. IP xxx.xxx.xxx.xxx is used as NAT outband at configuration Default.”

Not sure why this is an issue that Ubiquiti has left out there, but if you try and change the static IP address the UniFi controller will puke with the above error. Apparently there are a bunch of different workarounds, but the simplest (to me at least) seems to be logging into the USG directly (default IP is 192.168.1.1, but YMMV) and changing the WAN IP there first and then change the IP in the UniFi controller. The default login info is ubnt/ubnt, but this is likely changed and you can lookup the SSH login details on your UniFi controller and login with those.