HDFC Bank — Cash back not received

Hi i have purchased iphone11 (128 gb) on 11/feb/2020 from retail store with offer of rs 6000 cash back with in 120 days (Means with 4 emi completed) clearly mentioned on bill as well as confirmed by hdfc personal also now my all 6 emi completed its been more than 155 days i haven't received my cash back, seems like bank is making fool o[censored]s. I tried to contact customer service many times but there is no response even i went to hdfc bank gangtok branch but still no solution came out.
Please help me to get my cash back. Below mention are the complete loan details

Loan ref no: 55419756
Bill amount : 69900
Tenure - 6 months
No. Of installment paid - 6
Pending installment -0
Bank name - hdfc bank

Regards
Balwant