Airtel — mobile recharge not satisfied

Sir,
I am a new customer of airtel mobile. As a welcome gift they gave me 1 gb data free for 3 days. As i'm happy with their service i decided to continue their service. So as per today evening 4.22 pm' s sms of airtel regarding "congrats on your new 4g handset! Exclusive offer on rs399, 1gb everyday+ unlimited all local+std calls, 84days. T&c apply. Benefit only on 4g handset + 4g sim" i immediately recharge my airtel sim with 399/- rs. As per sms i uses 4 g moto e3 power handset & airtel 4g sim i must get the offer for 84 days. Right na?
But after recharging they said that the offer is not meant for me so i got only 28 days validity. How it's possible? Is my money has no value? If that offer is not for me then why they send an sms to me regarding this.
Please help me to solve my problem and do needful for making my money valuable.
Either i got 84 days validity or i will get my money back