The AT&T router has IPv6. Fortinet and I setup WAN and LAN side IPv6.
This is what was on the AT&T modem. I get one delegated subnet at home. The menu pages explain /67 to /69. I show you how to subnet IPv6 for your delegated subnet.
I created DHCPv6 on my Fortinet 60E, check it out in the menus.
After further testing on one of the Apache2 Web Servers resolves IPv6
Fortinet is checking into a way for the other web server to work IPv6.