Please help change firmware - UK vodafone

Please help, I have searched the forum but dont seem to get a solution that works!

I bought an unlocked S6 Duos some years ago and now need to put the official UK vodafone firmware onto it so I can use wifi calling

I have downloaded the firmware and tried to flash with ODIN but always get it failed when doing the image file. I have also tried to change the CSC code using the dialer but get MMI errors (even though it is connected to the network and I can make / receive calls).

I am sure I am doing something obviously wrong but cant figure it out! Any help really appreciated!
here are the details I have from my phone

Model SM-G920f
Active CSC code SIM
Available CSC codes.... lots including VOD (which is the one I think i need)
Bootloader version G920FXXU3DPBG
CSC Version G920FVFG3DPC1
Baseband version G920FXXU3DPB6


I have been trying to load :
Bootloader G920FXXS3DPF5
CSC G920FVFG3DPC1

Many thanks,

CHris

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