SM-G920T again locked after 6 months unlocked and work ok.??

SM-g920T back to me still locked after i unlocked phone before 6 months with Octoplus
now it wan,.t to unlock again
in phone menu it appears an SECURITY ERROR

This phone has beeb flashed with unauthorized software & and is locked .
call your mobile operator for additional support.
please not that repair return for the issue may have additional cost

any help what to do