For what it's worth, here is what AT&T has given me back after some fairly insistent "please escalate this appropriately" requests on my part:
I dunno what to do next about this. We aren't NTT's customer. AT&T is saying that it's not their problem. NTT says it's both their problems (generally). Kinda leaves us stuck not being able to do a whole lot, and thus the "please contact your ISP" request from the Lodestone doesn't feel like it's going to get us much of anywhere.I sincerely apologize for the inconveniences caused based on info found AT&T's services to get you to the ISPs of those you want to contact. Square Enix subscribes to NTT's services to allow you to connect to their servers.
Every traceroute I've seen shows that the latency issues begin inside NTT after AT&T hands off the traffic. NTT is not processing traffic properly from AT&T (and, based on the Square Enix posts, some other ISPs as well). So, the correct takeaway is the Square Enix should find a different ISP than NTT, not that any player should switch. A possible workaround would be the VPN solutions work is because it routes the traffic around the bottleneck in NTT's network near the AT&T consumer peering where a lot of consumers come in to a less congested connection.
Wish I could play the game with my kids right now without having to spend a bunch more money and time on VPN re-rerouting. (There's three of us playing, two on PS5.) And I'm not super-interested in the circus that is switching ISPs right now.