Below is a snippet of nginx configuration that will enable access to your UniFi Controller (eg. Unifi Cloud Key Gen2) using nginx reverse proxy.

server {
        listen 443 ssl;
        server_name example.com;

        location / {
                include /etc/nginx/proxy_params;
                proxy_pass https://IP_ADDRESS_OF_THE_CONTROLLER/;

                # WebSocket support
                proxy_http_version 1.1;
                proxy_set_header Upgrade $http_upgrade;
                proxy_set_header Connection "upgrade";
        }
}