I'm going with a prequel because it's basically backstory filler that can't really do anything major because it's ending is predetermined. At least a requel (remake masquerading as a sort of sequel) doesn't have to end a certain way. It could in theory break away from the original it's remaking.
Also lumping interquels (sequel to something older in the series but prequel to something else in the series) in with prequels since they're basically the same thing just sandwiched between to already released installments.
Also lumping interquels (sequel to something older in the series but prequel to something else in the series) in with prequels since they're basically the same thing just sandwiched between to already released installments.