iPad Trying to get iPad to work with mobile phone sim

[Spenspen]

I've been trying to get my iPad Air to work with a 3 mobile phone pay as you go SIM card,because it is 1p per Meg and the credit doesn't expire after 30 days. But the network doesn't like the sim and wants me to subscribe to a tablet sim contract. I have tried unlockit.co.nz and this sometimes works for a while but is very hit and miss.
does anyone have any other ideas
thanks