21-05-2015 12:46 PM - edited 21-05-2015 12:47 PM
For prepaid, roaming is activated by default.
There're two ways you can stay connected with Whatsapp, get DiGi Prepaid Internet Roaming(this can get really expensive if you're not careful), or use the local WiFi.
More info about Prepaid Internet Roaming.
But my advice is, apply their local tourist sim or rent a Internet MiFi device, which is much more affordable and usually comes with a tourist-in-mind plan.