Vodacom Blocking second SIM slots on mobile devices

I recently upgraded my contract. I looked at available devices. I decided on a Xiaomi Redme Note 9 Pro because it is a dual Sim phone. My reason for taking a dual Sim phone is because Vodacom can not sort out our signal in my area. I have been complaining for 15 years. I do Standby duties and in some areas in my house I have no signal. The plan was to add a MTN sim in the second slot. They have good signal in my area. The second Sim slot in the phone is Blocked by Vodacom. If I knew that I would have bought the phone directly from Xiaomi South Africa. I took the phone back to the store but they cant help.... I do not know who from Vodacom to contact. I am happy with the phone but not happy with Vodacom for not informing us that they block all second sims.