Welcome to the community and thanks for your post.
Sorry to hear you're not able to use your mobile data while in Greece.
I'm afraid we are not able to check anything account wise via the forum due to security. I've got some things to advise you though.
To use data abroad you would need to have data roaming active on your handset and have purchased a data travel pass from us. I've included a link here to our roaming page. This explains how to pick up a travel pass.
If roaming is working the data pass has been purchased and you still cannot use data I would recommend calling our mobile team on +44345 6000 789 so we can take a look at this for you.