Abnormal Data account

I have had a previous problem, for which they charged me for calls made from my Wi-Fi modem. MTN can verify when you remove the sim card from the device, and it was never removed. After calling at the Klerksdorp office who was willing to assist me, MTN insisted that I pay a further amount to obtain call details, which I know I did not make. I had to pay the account as I could not resolve the matter and were fedup with this. Me and my wife has been using the data with great care after a surprise month when we used Youtube and got an account for R700 on a R159 contract. I tried to register on the website to control my data usage, which is not user friendly at all, but they keep on sending the OTP to the sim card in the Wi-Fi device, which I am unable to obtain. Because of this I decided about 3 months ago to cancel the contract on the expiry date of 5 May 2015. Since I have given notice, I have been getting accounts of around R700 per month for data usage. We decided only to use the internet data for daily Whatsup and Facebook for the last few months, and was very surprised to get an account for R1738.43 for April. I have send a mail to customer service & are waiting for a reply.