Japanese Prepaid data SIM: Niseko, Rusutsu, Hakkoda and Kiroro
Coverage in Niseko
For full coverage detail check out https://www.nttdocomo.co.jp/english/support/area/
But as you can see from each screen shots there is coverage on the FOMA network that this SIM from DoCoMo uses. This SIM also works on the Xi network, which is even more extensive (only lets you select one option when looking at the coverage map).
Coverage in Rusutsu
Mobile data SIM coverage in the popular ski area of Rusutsu, with the Japanese data SIM from dataGO running on the DoCoMo network.
Coverage in Kiroro
Phone coverage in Kiroro ski area on the NTT DoCoMo network.
Coverage in Hakkoda
Phone coverage using the Japan data SIM from dataGO.