Disclaimers:
* I understand that Untangle does not implement some of the aspects of IPv6 needed in a typical setting (e.g. DHCPv6).
* I have read many of the forum posts relating to IPv6 including this one with dmorris mentioning he does this at home and it works.
* I am an IPv6 noob but I have tried to educate myself.

I tried to enable IPv6 on my WAN interface (using "Auto") and much to my surprise, I got a 2600:1700:x:y/64 address, I assume from my ISP (AT&T Fiber).

What I see on the WAN side:
* From the Untangle ping utility, I can ping 2001:4860:4860::8888 (Google DNS)
* From an outside ping service, I can ping the allocated WAN address (but no public LAN address).

On the LAN side:
* I checked the "Send Router Advertisements" box.
* I manually entered a 2001:4860:x/64 address as the LAN address (same prefix as the WAN address I got).

What I see:
* Local PCs get a public IPv6 address in the allocated /64 and can ping each other with those.
* I can always ping the public LAN address of the router (entered in the LAN config) from a local PC.
* I can never ping a public internet address like 2001:4860:4860::8888 from a local PC.
* If and only if the suffix I entered in the LAN config is exactly derived from the LAN MAC address, I can ping the WAN address from a local PC.
* If the suffix is any other value (0, 1, I tried a few more) I cannot ping the WAN address from a local PC.

I also tried to use the "Static" WAN config using the parameters gotten previously. I managed to have the UT Ping utility ping Google DNS, but only if I use the (WAN) link-local address as the WAN address. Still no LAN <> WAN connectivity.

I have exhausted my limited IPv6 knowledge at this point. Should this work somehow or should I just wait for DHCPv6 to arrive?