Although it's not the absolute cheapest, another option is to get a Verizon data-only prepaid SIM card. More info: https://www.verizonwireless.com/prepaid/

In short, the $35 and up tiers are active for 60 days instead of 30. Depending on data usage OP may be able to only re-up once instead of at least three times (once for the first trip and twice for the second trip). I use it with my Verizon Mi-Fi for the rare times I'm out of a T-Mobile service area and it seems to work pretty well. The iPad Air 2 also only had one model available for the entire world so there should be no problems with accessing their LTE bands.

If OP expects to use a lot of data, however, the other options mentioned in this thread are better.