Cisco DPC3939B and Asus RT-AC68u - no IPv6 on LAN?

I've been banging my head against this for a few hours now and have only been giving myself a bruise and a headache.

Here are the facts:

I'm a new Comcast Business Class customer with 1 static IPv4 address. I transitioned over from consumer-class as I wanted to be able to host my own email server.

The gateway is also receiving a IPv6 address, which is dynamic if other posts on this forum are to be believed.

The DHCPv4 server on the gateway is switched off; I've made the gateway into as much of a "dumb bridge" as I believe possible. My RT-AC68u is doing the firewall/NAT/wireless/DHCP on the IPv4 LAN side. The RT-AC68u is directly connected into the gateway on LAN1.

Here's a screenshot of the current IPv6 settings on the RT-AC68u:

As you can see it is not showing that it has a LAN address for IPv6. If I SSH into the router, it shows it has an IPv6 address from the gateway.

I've tried disabling DHCP-PD and setting it up as a second LAN device, but it mostly doesn't work - when my Windows 8.1 box says that IPv6 has Internet access, it apparently can't route packets beyond itself.

My PC's secondary NIC, when connected directly into the LAN2 port of the Cisco DPC3939B, receives an IPv6 LAN address from the gateway, no problem, and I am able to ping and otherwise conduct traffic to and from IPv6 Internet sites.

As a workaround, I can use a Hurricane Electric tunnel on the router but I'd prefer to use native support where I can.

Any suggestions? Other posts on this forum seem to suggest that the Netgear gateway is the way to go for native DHCPv6 on a LAN at the moment as the Cisco gateway is currently incapable of doing the correct DHCP-PD handoff.