Just guessing as my ISP delegates IPv6 prefix using DHCPv6:
/ipv6/address
add interface=sfp1 address=2a05:xxxx:yyyy:zzzz::ww/126 advertise=no
/ipv6/route
add dst-address=::/0 gateway=2a05:xxxx:yyyy:zzzz::gg/126
/ipv6/pool
add name=myIPv6pool prefix=2a05:xxxx:y:zzzz::/56 prefix-length=64
After that assign addresses to LAN interfaces of router:
/ipv6/address
add address=::1 from-pool=myIPv6pool interface=<LAN interface> advertise=yes
After a short while, LAN clients should start receiving RAs and construct their own IPv6 addresses inside same IPv6 /64 subnet.
You can affect a bit more the selection of prefix (and address) for each individual LAN interface, with property address you essentially set the least significant bits of address (so it has to start with ::) while the most significant bits are filled automatically by router by pulling prefixes from pool. Make sure that pool has prefix-length set to 64 (and nothing else), because this governs then the size of prefixes pulled out of pool (and has nothing to do with pool size). Using /64 subnets is more or less standard and one has to know their stuff to go for anything else.