Security Error & Network Lock

I bought this Samsung Galaxy S6 Edge SM-925T unlocked phone, worked for 6 months fine. Now it is suddenly started :

Getting these errors:

Security Error:.

This phone has been flashed with unauthorized software & is locked,

Call your mobile operator for additional support.

Please note that repair/return for this issue may have additional cost.

Network Lock:

"This device can be remotely unlocked if eligible. Please use the "Device Unlock" application to unlock the device, Make sure the device has an internet connectivity to use the "Device Unlock" application.

Updated the firmware to latest via Kia's.

Reset the phone to factory default settings.

Your advise is greatly appreciated.