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.