In the [...] mobile application, select "Dropbox" as the synchronisation target in the Configuration screen [...]. Then, to initiate the synchronisation process, click on the "Synchronise" button in the sidebar and follow the instructions.
I recently started using Joplin, begnning on my Macbook and importing notes and notebooks from Evernote.
I also have a problem with iOS and iPadOS using Dropbox. Joplin syncs fine between my Macbook and iMac. Notebooks and notes match up, and new notes created on one sync on the other.
On my iPad, the notes from the time of installation (August 27) synced. However, my notebooks were not added (but tags were). Also, anything created on my iMac after August 27 have not synced. Similarly, a test note and notebook created on my iPad have not synced to my iMac.