Update: for a newer version of this information set within the Config Tree portion of the web interface of the EdgeRouter 4, read this post, with pictures and all! 🙂 https://davidwesterfield.net/2021/03/enabling-ipv6-prefix-delegation-on-att-internet-for-a-second-firewall/
I’m archiving this information for future reference because I (or others) may need it. This was extremely helpful in getting AT&T’s allotted IPv6 subnet(?) (properly called: delegated prefix) setup in my EdgeRouter 4, although I wound up having to use the web interface and configuring the same settings within the Config section. Without further ado (or a whole lot of ado below) here is Bradley Heilbrun’s explanation.