IPv4 is provided using DS-Lite or MAP-E depending on the provider.
I'm using OpenWRT and paid for a static IP so I had to manually configure all the details for the MAP-E tunnel in OpenWRT myself, I think typically the routers sold to consumers pick up the configuration automatically somehow.
Woah, MAP-E allows static v4 (and presumably inbound connections)? That seems neat and much better than DS-Lite!
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.