MTN misleading to sign you up on contract

MTN has contractor called Mando. MOndo signes up people on Mtn contract etc.so after telling them I got a phone with 1 SIM slot and i dont want to change my number and i dont want to be swaping SIMs all the time. They told I can do a SIM swap and keep my number on contract account And I was ok with that. When I received the SIM cards they were already activated so I couldn't do SIM swap. So obviously because what we agreed on ddnt happen I was unhappy. So i asked them to cancel the contract. 6 month later still not resolved. MOndo one time said I can discard the SIM but mtn is still debting for Sims that i ddnt even use. Can someone help. This has a bad effect on my credit score. MTN PLS CANCELL MY CONTRACT.