Wrong billing

I app**** for a huawei router, it was on special for R369 for a 40 gig day and 40 gig night per month.

I first bill came and it was for the amount of R900 which we could not understand, we only use the wifi for wattsapp and facebook.

I tried to phone telkom but no one could assist me. On the account statement it states gig for the amount of R900.

I am very disappointed that this happened, this cost more than the uncapped wifi that R600 per month.

I would like someone to assist us please.