If you are intending to configure an `AAAA` record, then configure that address as a static IP address. It is best to do that on the server.   OpenWRT uses `dnsmasq` for DHCP and DNS. `dnsmasq` can be configured to supply a static IPv6 address if the client using DHCPv6.

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.