I recharged through the MyJio app on 21st August at around 9 AM using my new Axis Neo Mastercard credit card for ₹479. The merchant was listed as 'PTM*RELIANCE JIO' with an MCC of 4900 (Utilities).
The transaction settled yesterday, and today, 23rd August, I received ₹300 cashback.
Please note it doesn't have to be your first transaction, but rather has to be your first utility transaction within the first 30 days of card activation.