In this situation, at any given time you can cancel your Premium subscription in the Apple App Store. If you do not cancel it, Roblox will continue to bill you—as customer care has stated in my reply above.
However, to get a refund you need to go to Apple’s official “Apple Refunds” website where you can report a case, get a refund, etc.
Quite simply, Roblox’s product, the Premium 450 subscription is never given to you even after your payment is processed. This leads me to believe that this is a problem on Roblox’s end because Apple fully processes the payment.
Note: Using alternatives like Credit or Debit Cards, Roblox Gift Cards, etc. should all work.