Shocking Service & Billing!

I signed up for cellc 2gb(day) + 2.5gb(night) special, for a monthly fee of R119 with a usage limit of R369.61 as per my contract. The modem is hardly used as it surprisingly runs out quickly so have been using my backup (Telkom modem). On 25th Dec I received notification that my cell c data was on 500MB and on 28th Dec is stated that it had depleted. During Dec month the modem was mostly used at night to take advantage of the night time data and now to my surprise I received a bill stating I owe R1628.47 yet in black and white I signed the contract stipulating the limit of R369.61. How on earth is it possible to even owe that amount? I would like to request for this to be adjusted as it is clearly dishonestly from your side taking advantage of costumers just as the company as is going down the drain. I will not be paying this as it is a *******! Please update my invoice and I'm not interested in upgrading.