Flipkart - Refused Replacement and refund for my phone in flipkart in return period itself

Order ID - OD******************

My Phone Mi a* is getting turn off frequently. (Doesn't turn on itself).That too while using. Even if I turns on, after few minutes it goes, the process repeats so. If I connect charger and turns on, eventually it working smooth. This happens in the morning time(I mean after a long idle time), But after few hours, phone getting turned off itself. If it turns now, working smooth.
I did factory data reset and troubleshooting. Also I updated the phone to latest Android **. But still the problem persists. This is my new Phone (just a week).
The problem started from time of unboxing itself. I contacted flipkart for replacement. They sent a technician from "jeeves", he just restored my device and gave to me, after sometime I got mail saying "Your replacement rejected". As the problem persists, I went for refund. Again that too rejected without any reason(They didn't even contacted me) saying contact the brand.I bought phone from flipkart,they delivered me defective product, Not the brand.I do have video evidence of device malfunctioning. **/**/**** is the last return date. I desperately need my replacement or refund. I fed up calling customer care.