Move online first react native app to offline first with mongodb realm

Hello,
I am having challenges migrating my online app (node backend) to be offline first.
I have successfully connected my app to the app services and enable device sync. In the users section under app services, i have registered users and am able to log them in in my app.
However here is where am stuck, I want to link the user to their data which they had while the app was online.
What do I mean?
While the app was online they would login through a rest api endpoint, right ? and as per the schema each user would their data.
In mongo realm I did not find a way to login the user with the data that is already stored in mongo db. The only way was to create users under the user section in app services.(please correct me if am wrong here)
This works and i can log them in, but now i want to access the data in mongo db so i can display that to the users.
How can achieve this?
please any help will be appreciated, Thanks

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