Firstly, ensure that your international data roaming service is activated. Once that is done, you can manually set your phone settings by following the steps below:
Android:
Step 1: Go to “Settings”
Step 2: Select “More Networks”
Step 3: Select “Mobile Networks”
Step 4: Select “Network Operators
Step 5: Select your preferred network
Step 6: Turn on “Data Roaming”
iPhone:
Step 1: Go to “Settings”
Step 2: Select “General”
Step 3: Select “Cellular”
Step 4: Turn on “Cellular Data” & “Data Roaming”
Step 5: Select “Carrier”
Step 6: Change “Automatic” to "Off"
Step 7: Select your preferred network