Thank you bluepoole. Changing my 4G settings to Data Only solved the problem. Strangely neither any of my colleagues nor I experienced this issue with iOS 11. Not sure if it's a Vodacom or iOS issue, but at least it's working.
Thanks creeper. My default setting is roaming off but I have also enabled it now but unfortunately it makes no difference when I disconnect from the Wi-Fi network. Is there another setting I'm missing?
My iPhone 6 Plus is currently on iOS 12.1. Since updating to iOS 12 I receive the following message:
"Could not activate mobile data network"
every single time I disconnect from a Wi-Fi network.
I have to manually enable and disable flight mode to make it work. This has never happened...