G920F Cant Write Cert, Modem downgrade Fail


Trying to downgrade modem on this to unlock/write cert but any modems I try give unknown baseband.

Anyone have any suggestion, the device has IMEI 3500..

 Please reinstall a complete stock firmware and try again!
Model: SM-G920F
Sales Code: BTU
Country Code: UK & IRE
Timezone: Europe/London
Android Version: 6.0.1 [MMB29K]
Build Date: Mon Jul 4 11:04:47 KST 2016
PDA Version: G920FXXS4DPG2 [july 2016, rev2]
CSC Version: G920FXXS4DPG1 [july 2016, rev2]
Board Platform: EXYNOS5
Serial Number: RF8G60Y34RR
Battery Status: 3.75V (21%)
Network Type: GSM
Knox Version: v30
Warranty Bit: 1
Security Patch: 2016-07-01

Please help