Got to an ATT store, ask for the stand alone 20/mo Ipad plan, I just used the IMEI number of the ipad in the store. You will leave with an activated sim card. This is a post paid plan that requires a credit check. If you don't want to go that route you can sign up for the prepaid plan, but it is 35/mo and is done on line, over the phone or in a chat.
Good luck, tell me if you find this plan. The options above have a deprioritization after 22Gb. This means you will be put in the back of the line but have throughput still. Your ping just goes up. This is only if your tower is busy or congested.
Get a Nighthawk. You want the band 14 capable version. Put the SIM card in it and go. Run this up in the air, do not use external antennae, this ruins the 4x2 MiMo capability. Power it however you like, via a PoE>DC splitter or plug it into the wall. Run it w/o the battery. Do not use over a Tb of data or you will have a bad time.