not getting incoming calls

I took out a cell phone contract with you guys 2018 September, i decided to pay it off last month, and choose to keep the number as prepaid, the cancelation was done on the 12th of March 2020, ever since i cancelled my contract i am not receiving calls, when i call my number "it says number does not exist", i called mtn several times, i was told to dial #330*0000# and #35*0000#, and my phone will be okay, even now am still not getting calls, am missing important calls because of this, i cant call the customer service number anymore because they are not helping me with anything, they tell me to do the same thing am doing and it's not working,please sort this thing out