why does my iphone 6 shows searching even after it has been unlocked

I bought this iphone 6, checked its lock status and found it clean unlocked. But when i insert my sim, it works for a day or 2 then it shows "Searching" or "No Service" on network bar. I have updated it to the current IOS but immediately I update the IOS it works for a day or 2 then it shows again. Please help me out because I don't know what the problem might be. And when you open Settings and About, it shows the SIM carrier network I have inserted but cant make calls and access the net.

The Modem Firmware is 6.55.00

Thanks.