It’s supposed to be (and usually is) automatic, but there are occasions where the WiFi may cut out and the phone begins using paid data. To overcome this, open the Republic app, and in settings (the gear icon) choose “Data Tools” and turn “Data Freeze” on. This should allow the phone’s continued use of “unpaid” data, which it needs to operate properly, while suspending the unnecessary use of paid data.
Since I’m usually in an area with WiFi, I leave data freeze on most of the time, and only turn it off when I actually need cell data. That keeps my monthly data usage at 500MB or less.
You can also add the Republic widget to your home screen which will show whether Data Freeze is activated, and (in most instances) turn it on and off from the home screen.