Pay for 5G data contact but only get 500M

A year ago I signed up at Telkom for a 5G data contact on a special they dad promoted, and from the first month only got 500m of data. I have been back at their shop in Greenstone as soon as I have discovered the problem but they got no interest to rectify the problem. The sales person that have feed the contract details into the computer did admit he made a mistake but he cannot fix it. We phone some body that requested me to send the original 5G contract to her ********** by e-mail witch I did. After hearing nothing, I went back to the shop more than 5 times and still only get 500m data a month. I cannot stop payment then I will sit with a bad credit record. There is still one year to go. I cannot recommend any one to support Telkom. Jack