I spent a few days getting this to work and thought I would post it here in case it helps anybody. There are a few threads on this topic but I believe they are using an old version of the firmware so the screens are different.
Step 1 - Enable IPv6: Go to Configuration -> System -> IPv6 and Enable IPv6.
Step 2 - Create DHCPv6 Service Request Object: Go to Configuration -> Object -> DHCPV6 and add a service object with Request Type = Prefix Delegation and specify your WAN interface. I called my service object "TWC_Prefix".
Step 3 - Configure WAN Interface: Go to Configuration -> Network -> Interface -> Ethernet and edit your WAN interface. Click "Show
Advanced Settings". Enable Interface. Enable IPv6. Check Enable Stateless Address Auto-Configuration (SLAAC). Select "Client" for DHCPv6. Check Request Address. In DHCPv6 Request Options click Add and select "TWC_Prefix".
Step 4 - Configure LAN Interface: Go to Configuration -> Network -> Interface -> Ethernet and edit your LAN interface. Click "Show
Advanced Settings". Enable Interface. Enable IPv6. In Address from DHCPv6 Prefix Delegation click "Add", select "TWC_Prefix", and specify a suffix of ::1/64. Check Enable Router Advertisement. In the Advertised Prefix from DHCPv6 Prefix Delegation box click "Add", select "TWC_Prefix" and specify a suffix of ::0/64.
Step 5 - Create a firewall rule to allow ICMPv6 into the router. This is necessary to exchange router information.
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