It's only going to be eligible on things paid through Roku (i.e., Roku Pay). If you've rented through Fandango before and the payment had nothing to do with "Roku Pay" and the payment method tied to your Roku account, then I would imagine that you're not going to be able to use it there. If your previous rentals did use your Roku payment method (check your previous purchases at https://my.roku.com), then it will (or should) be automatically applied to any new rental probably without any indication when you actually make the purchase. Maybe someone that has used it successfully can confirm that.
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