To configure your Device for International Roaming, follow these steps:
Manual Configuration – Go to Settings > Network Selection > select 'Manual'. Wait while your device does a search for available networks. Then, make sure to select the correct network to enjoy the best rates when roaming
Automatic Configuration – Turn on your device and wait for it to connect to any available network. If your phone does not select a network automatically, use the Manual Configuration
To turn on Data Roaming:
For iPhone users
Go to Settings > General > Mobile Data > slide the Data Roaming slider ON/OFF.
Steps may vary depending on the iOS version on your iPhone.
For Android users
Go to Settings > Mobile Networks > Tick or untick the box beside "Data Roaming"