Sim Swap to my contract cell phone using my old number to my contract sim car.

I bought a cell phone contract through Mondo. upon receiving my contract sim I requested that I need to use my current cell phones number instead of the new sim card number. like eg to do swim swap and still use my cell phone number a as contract than the new sim.. because I cant forfeit my old number and use this new number. Good I contacted Mondo because I bought this Contract Phone through. They said to me I should Contact MTN. Once I called MTN CONTRACT CENTRE on *** they said the cant help me with linking my old number to my contract number, they said only at store level they can do that. Good I went to South Gate Mall in Gauteng MTN store, lucky enough the Mall there has two MTN stores, both stores told me they cant help me I must call MTN to request MTN swap, long story short I am been taken from pillar to post. what's difficult cancelling my contract sim and linking my old numbers soo that I can enjoy the benefit? now that contract sim I am not using it and it has DATA AND ALL THAT BENEFITS. this is my last resource please help me to use my old numbers as my contract please.