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.
