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That should be relatively simple dispute, so online would probably work because you're not disputing any negative info
Reason for dispute is "paid in full"
There is a possibility this is mess up on TU part, because identical data is provided to all three CRA.
In any case, you already did the direct dispute step (with a lender)
Now you file dispute with TU. If that fails, CFPB complaint time.
Good luck