State Bank Of India [SBI] — refund on debit card

Address:

Seawoods grand central mall, Mumbai City, Maharashtra

I purchased the cellphone on date 25 June 2019 at oneplus showroom, grand central mall seawoods navi mumbai and paid bill by using SBI debit card. There is offer of 1500 cash back on purchase of 25000 and above using debit card. Oneplus murchant have processed the amount after 2 days of purchase but refund not received yet. it suppose to be in account within 90 days of period and its almost 145 days and still not received the cashback.
I called to customer care of SBI bank but nobody able to give me the information regarding my cashback not even able to suggested other options.