I am so sorry for the bad experience.
In my tests I was able to recover from iCloud without any issues.
Just reinstalled the app on one of my phones and enabled the Sync - all data was downloaded again.
A few possible explanations:
- The sync (upload) didn't finish on your old phone
- You were using two different iCloud accounts on those devices
- You deleted the iCloud data for Diarium
Did the sync finish successfully on the old phone?
What exactly happened when you initially set up the sync on the new device?
I assume you don't have access to your old phone any more?