Adding a the Ubiquiti Unifi Security Gateway (USG) to my existing setup was slightly more involved than I had hoped however it was fairly idiot proof. The issue is if the controller is on a different subnet to the USG then it will fail to adopt the device in the controller.
Follow the guide before replacing the existing router.
It’s best to start configuring the LAN IP where it has access to the rest of the LAN. The part that caught me out was in Change the USG LAN IP Before Adoption section and after step 4 but before step 5, unplug and plug the network cable as stated however do not turn the power off to the Security Gateway. If you do all settings will be reset.
Go straight onto the adoption stage because it’s only once it’s adopted the LAN IP settings will stick.
It’s a silly mistake but knowing how to complete the adoption process was the hardest part of the setup process.