device is flagged for fraud

Hello, I purchased IPhone 5S from AT&T . After 2 years of contract, unlocked the phone. I had to move to India and had been using this phone for more than 2 years without any issues. Suddenly, few days back the device was not able to connect to carrier network. It was displaying "Searching for Network'. I replaced the SIM in other device and was able to use ( ruling out any SIM related issue). I used another SIM in my Iphone but had the same issue.

I reset my IPhone to factory setting but was unable to activate the device, got an exception " unable to connect to activation server" . Then, I verified my device status in AT&T and placed an unlock request again . However, received an email stating that my device has been flagged for fraud. I had a chat with AT&T and my local carrier "Airtel" in India. Both mentioned that they do not see an issue at their respective end and advised to check with Apple customer service.

Can you please help activate my IPhone?