Make sure to complete the following:

1. Turn on data roaming on device settings.

2. Connect to our partner operator on device settings. (Only for single destination roaming add-ons.)

How to connect to partner operator?
 

iOS: Settings > Mobile Service > Network Selection > Turn off Automatic > Select partner operator

Android: Settings > Connections > Mobile networks > Network operators > Turn off Select automatically > Select partner operator

Check order of usage for multiple roaming plans

Unused roaming data after expiry