Unfortunately, I cannot solve this from my side. Diarium Pro purchases on Android are handled only through Google Play, and the available payment methods are controlled by Google.
Diarium does not use its own account or separate licensing system, so I cannot manually activate Pro or accept payment in another way.
If payment through Google Play is not available, Cloud Sync unfortunately cannot be unlocked. As a workaround, you can transfer data manually using backup/export and import, but this is not the same as automatic sync.
