If you are intending to configure an `AAAA` record, then configure that address as a static IP address on the host that requires a static IP address.  DHCP can be used to serve static IP addresses, but it is a poor second choice solution.  

OpenWRT has used `dnsmasq` for DHCP (v4 and v6) and DNS. `dnsmasq` can be configured to supply a static IPv6 address if the client using DHCPv6.  The latest version of OpenWRT appears to use `odhcpd` for some if not all DHCP services.  

Use the firewall to open ports from the Internet to the server.  (The address you assigned will be its address from the Internet as well as your LAN.)  You should be able to connect locally to the server without opening any ports on the firewall.

`odhcpd` does support static IP address by adding a `host` section to its configuration.  I haven't my router to the lastest OpenWRT version, so I am using `dnsmasq` which can also provide RA and DHCPv6 services.  You should be able to specify the static IP address at the bottom of the DHCP and DNS configuration page.