ZTE Cryfi

I bought a ZTE Mifi at Cell C Cresta on 16 December 2017. I was informed that it works like a bomb, is super fast and up to 10 people can use it at the same time. I did not take it on contract but on a pay as you go, as I would prefer to control the usage. It unfortunately is not what I was told it would be. It is super slow and, not even two people can use it at the same time. When one is surfing the web the other persons data lags. It eats more data than my normal Cell C phone usage on the web. When I took it back asking for a refund or credit note I was informed by a trainee manager Mr Naidoo, that they would first have to send it to be checked out. I don't want it "checked out" I would like to have my money back as I was sold an *********** piece of electronics. I also do not want a new Mifi ZTE. Please can you assist. Thank you.