Hello,
I think now we can say how to unlock the Classic and World XI Team online. I will explain it i a few steps.
1. Unlock the Teams offline.
2. In "my Fifa" under profiles you create a new profile.
3. When you choose the team, click "change fow" in the left bottom.
4. Go to 4,5+ Stars Teams, there you will find the Classic XI and World XI Team.
5. Select the team you want as your favourite.
6. Now you can play online with it. But be careful(!), when you select another team in the session, you are not able to choose the classic teams again.
I hope i could help you,
Best regards,