Ripp off by Telkom wifi bundle(dishonesty)

Telkom sold wifi bundle to me for 20GB day and 20GB night which makes it 40GB in August, at a rate of R269 for 24 months. I started using their service in August and i have a contract with which the amount as mentioned. Today 26 September 2018 i received a statement, stating that i owe R538.00 of 10GB day and night which makes it 20GB...so in other words this could mean this was just a catch for us to sign the contract of 40GB according to their "special" to lure customers so that they can make money. Can someone please assist in this matter. The contract was signed by my husband.