I am using Mac version 2.10.19 and iOS 12.11.3 syncing with Dropbox
Both devices are devices provided by work and we switched to a new MDM, therefore both devices were wiped and I restored from a backup (at least in the case of my iPhone). Then I launched Joplin and the only notes in there were the standard beginner notes. So I reconnected my Dropbox and performed a sync but no notes came over, only the standard beginner notes.
So, I did the same with my iPhone. Now I just have two copies of the standard beginner notes.
I have the standard Dropbox account. I have not touched anything else hoping the data was not overwritten and I can get my notes back.
I'm afraid I can't help directly with troubleshooting, but in my experience Joplin is backed up in iCloud by default, including all its data. That's why I was so surprised one or two years ago when my storage quota was up 1.6 GB all of a sudden (exactly the size of my Joplin data folder back then).
Now I'm surprised that restore from iCloud didn't work for you - maybe anyone reading this has tried to restore Joplin from an iCloud backup before. First thing I did back then was to turn off iCloud backup for Joplin, because end-to-end-encryption for iCloud (called Advanced Data Protection) was not available at that time.
Do you have a notion what size your notes and resources directory was, and can you find Joplin's sync folder in Dropbox?