What's the point of Airdroid's "Find phone" feature if phone is not on the same network
You can create an account with Airdroid and link your phone to that account. It'll then use a 3g/4g connection if WiFi isn't available.

Alternatively, if you're looking to use it as a security feature, if your phone is linked to your Google Account, you can use Google's Device Manager to wipe/Find/Lock/Make it Ring remotely.