Japanese ISPs are, by and large, assholes, doing whatever they can to force their shit routers on customers. Even NTT, the traditional monopolist and the most regulated one, which by law cannot refuse to let customers use their own hardware, hides configuration parameters to make it as difficult as possible. Thankfully, some enterprising Japanese users have figured out configuration parameters for NTT’s Flet’s Hikari (フレッツ光) service and posted them online. I have aggregated this knowledge, in English and in one place for easy reference, here.