Which provider are you using? I'm curious about this since there are not many OpenWrt guides for getting connected in Japan. Is your config similar to this write-up? https://github.com/fakemanhk/openwrt-jp-ipoe

I didn't need to do any configuration for DS-Lite or MAP-E, as DHCPv6 with a configured prefix got IPv6 working, although DNS is still broken when turning off IPv4 entirely.