Penalty fee for a product which I own

Don't ever purchase anything from vox. I have bought a Zyxel router through their website and made a once-off payment of R800. 4 months later i am surprised with a statements attached to an email charging me a R770 penalty fee for not returning the product. I was never notified that i should return a product which I OWN. It was never stated that the product which I bought should be returned after a specific time nor was it ever stated ANYWHERE that the product which I BOUGHT is being rented or needs to be returned. On top of it all, i have been waiting for a few days now for anyone to get back to me via email after i tried to discuss the matter through customer service. I honestly feel like I'm being cheated by this company.