HDFC Bank — Cashback not received

I bought iphone 11 from faridabad merchandiser. I was updated that i will receive a cashback within 120 days however its been 6 months, i am waiting for the cashback. I checked with the shopkeeper from which i bought the phone. He said that i have to contact the bank. I tried approaching custoner care however ivr not let me through. I never expect such hassel from the big service provider like hdfc bank.