java.net.UnknownHostException error REST API

Hi,

I'm using the open source (free) SaopUI and I am attempting to connect to the api: https://api.genderize.io

When I run the tests with internet connection it works fine but when I run without internet connection I get the error: java.net.UnknownHostException.

I've been told that APIs shouldn't be affected by internet access, I am relatively new to REST APIs so can someone confirm whether internet access is required for REST APIs and if so why my test isn't working when I turn my internet off?

Thanks