Welcome to the forum, peteinhawk!
I‘m only also an ordinary user, but perhaps I can give you a start:
On each device Joplin saves it‘s database locally, e.g. on your laptop and on your mobile. So what you did in the past is synchronising these local versions of your database with the dropbox version.
Now, with a new mobile device, nothing between laptop and dropbox needs to be changed.
You just can make your new phone synchronizing with the dropbox data.
But before doing that, it is always strongly recommended to backup your database, if not done already. For that, you can use Joplin‘s „simple backup“ plugin for the desktop version.
After that, you can connect your android with dropbox in Joplin on a path similar to this: configuration -> synchronization -> synchronisation target -> Dropbox…