The client's IP address is retrieved from under the $_SERVER global variable, normally from under property 'REMOTE_ADDR'. However, different platforms may require to use a different property name to retrieve this information.
Cloudflare might use 'HTTP_CF_CONNECTING_IP'
AWS might use 'HTTP_X_FORWARDED_FOR'
The property name to use can be configured in the "Plugin Configuration > Server IP Configuration" tab on the Settings page: