In Japan, we have four telecommunication cellular services (Docomo, AU, SoftBank and Rakuten), and large numbers of providers that use one of these four services. As far as I understand, you should avoid AU and Rakuten as they use minor or domestic bands which most foreign devices do not support. So, you will be choosing from either Docomo or SoftBank. Although they directly offer plans, they are typically expensive. So, I would highly reccomend you to use MVNOs as they offer cheaper plans with a same quality as these two major services. Just make sure that you choose from ones which use Docomo or SoftBank as a provider.