Diarium version: 2.8.16
OS + version: 15.2.1
When I was on the plane's WiFi, every time I opened the app, it would pop up that it's uploading the local database (to Dropbox), and would stay on that indefinitely, with no way to access the app. The WiFi otherwise worked for iMessage, emails, and web browsing.
I wouldn't mind it being unable to sync then, but it should let me access the app, either by letting me dismiss the syncing popup or automatically dismissing it after a while.