Sounds like you definitely need the firmware update, I went to my local Stealers and they said it could not be done....
So don't waste your time with the Main dealers.
It can be done, I've had mine done and now have the latest maps 2018-2 installed all works fine 7 digit post codes now work.
I had a coder round to do mine, It meant a full car firmware update, every module in the car needed to be updated as they all need to run the same version of firmware (At least thats what he told me),and took him over 5 hours to do, and he had all the needed equipment.
It is not something that can be done via a laptop and lead. He had to gain access to the fibre optic ring to do it.( with the same kit the dealers use)
So it is possible but not a DIY job for sure.