I've tried connecting BE85 directly to SoftBank/NTT East ONU, and:

- IPv6 works out of the box (deco app upstream setting: MAP-E)

- IPv4 doesn't work (must be SoftBank's deviation from standard, there are blog posts about that)

This is a great business opportunity too, because SoftBank has tons of customers, we're talking about premium tier internet (1G or 10G fibre) and Japanese customers can and happily do pay for good service (simpler setup, pretty device like Deco).

SoftBank currently offers a single "mesh wifi" option, but I figure newer Deco devices are much better.