is the Unifi Cloud Key - I just have one for all the sites to connect into, with dynamic DNS for that node.

I run a Unifi Controller on my Raspberry Pi and have it updating dynamic DNS so I can easily connect to an L2TP VPN to access my network away from home

I use the Dynamic DNS hostname for this, so in case things change (Modem reboot, ISP re-ips) , as long as I update the DDNS IP to point to the node with the Cloud Key, the remote USGs and APs will always find the controller