Similar issue, only with iCloud and not Dropbox.
DT3 on MacBook Pro - sync - DTTG iOS (iPad Pro)
In the DT3 preferences pane for sync there is a status update on the Global Inbox that says last sync was 5/8/19 although I’ve been using both devices daily since.
I tried deleting all data from DTTG and reinstalling the databases there but this hasn’t fixed the sync issues. Now the status on the Global Inbox is “Not yet synchronized, merge with remote databases” The spinning wheel is spinning continuously even though there are no documents in the Global Inbox for either device.
DT2.11.3 syncs just fine.