For some reason I decided to look into IPv6 again. Initially for AT&T since I was in the router page and saw it as an option that was disabled but not changeable. Then I logged into the router I have at my kids' house. I ran tcpdump and saw this:
root@OpenWrt:/proc/sys/net/ipv6/conf/eth1# tcpdump -vvv -i eth1 ip6 tcpdump: listening on eth1, link-type EN10MB (Ethernet), capture size 65535 bytes 18:36:53.462003 IP6 (class 0xe0, hlim 255, next-header ICMPv6 (58) payload length: 32) fe80::7281:5ff:fe2c:bbd9 > ff02::1: [icmp6 sum ok] ICMP6, router advertisement, length 32 hop limit 64, Flags [managed, other stateful], pref medium, router lifetime 1800s, reachable time 0s, retrans time 0s source link-address option (1), length 8 (1): 70:81:05:2c:bb:d9 0x0000: 7081 052c bbd9 mtu option (5), length 8 (1): 1500 0x0000: 0000 0000 05dc
There's no prefix in there. Hmm. Looking into this, the Flags "managed" led me to believe DHCPv6 was required, and it was.
opkg install wide-dhcpv6-client
Set that up, voila! Why isn't this advertised? Maybe newer modems do more with it, but my Ubee U10C018 DOCSIS 2.0 passes it thru just fine.