False promises from MTN

I was advised by MTN Sandton City branch to upgrade my Apple 7 Plus mobile device 256GB to Huawei Mate 20 Pro 128GB just before Christmas 2018. Naturally I had an issue with Huawei only having 128GB but was told that I could get a 256GB Nano Memory Card inserted into the spare SIM slot effectively trebling the memory. These would be available from MTN (or widely a available elsewhere) on 7th Jan 2019. I went for the Huawei on this condition. Today is 28th Jan and absolutely no joy. No retailer has even heard of this card. MTN are not responding despite many visits to their SC store and I can't get hold of anyone at Huawei. Frustrating, annoying and unacceptable to say the least. Please assist...