SIM is not working after updated to IOS 8.4 for iPhone 6

Hi,

I'm using iPhone 6 and it is factory unlocked.

Today I downloaded IOS 8.4 firmware from here https://ipsw.me/all#iPhone72 and restored my phone vis iTunes. I was using IOS 8.3 eariler.

Surprisingly, now it shows:

The SIM card inserted in this iPhone does not appear to be supported.

The SIM card that you currently have installed in this iPhone is from a carrier that is not supported under the activation policy that is currently assigned by the activation server. This is not a hardware issue with the iPhone. Please insert another SIM card from a supported carrier or request that this iPhone be unlocked by your carrier. Please contact Apple for more information.

My question is if it's carrier is locked then how I used my SIM earlier?

Then I downgraded to IOS 8.3 and same issue exists.

Need help to fix it.

Thanks

Nazmul Haque