Topping up International Pre-Paid Data SIM Card if credit expires

I've just ordered and received a Pre-paid international SIM card with 250MB data included. It is JT branded, and I have installed the JT TravelApp. My question is: if the credit expires or gets used up, can I use the app to add more credit without a WiFi connection? Or do I need to find a WiFi connection or use an internet cafe to add more credit?