UNISA not responding to emails

I am a student at UNISA I have missed my exam last year due to medical conditions and I have sent them my Doctors Certificate but I haven’t received any respond from them. I have sent several emails not a single respond regarding that. I have a good year mark I was hoping I would write an exam again but since there’s no reply I don’t know what to do and I can’t afford to re-register for the same subject again.