Download and install the Dropbox app onto your mobile device and sign in in that app;
Enable viewing of the files via Edit on the FD Mobile app or Files app.
Also, you may want to refer to this article from Apple on adding and enabling third-party apps and using the File app: https://support.apple.com/en-us/HT206481
If the Dropbox app is already installed, make sure it is enabled via Browse tab Edit:
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https://fd.support/3J10Usz