Which SIM card works with Mofi 4500 the best?

[Pupper3]

Hello, new here! So I have a mofi 4500 that I still don't have working right. I was under the impression I could simply pop in my sim from my iPad but tried and didn't work. I have service through sprint. Also thought I could use the sim from another active phone and followed the steps given by the mofi tech support, only to be told at the end that I needed to purchase some other special data only SIM from sprint and pay another $70-80 per month. Is there a cheaper way to go about this? A cheaper plan out there? Does the mofi with with a hotspot sim? I also see some sim and plans on eBay but I'm hoping to get an unlimited plan if possible, unless there is some way to still get unlimited data using the mofi even if the plan isn't unlimited? Basically, for those that have a working mofi, which SIM and plan did you have to purchase? TIA