Not Square-Enix support, but I'd be curious how similar/different the two traceroutes are. Intermediary devices are allowed to inject an RST packet as if it came from the destination when a connection 'needs' to be reset, after all; from the endpoints, it looks like the opposite side of the connection sent the RST. So if there's something along the route for AT&T that is throttling connections for some reason, it could likely show up as an RST from the server, while if Verizon doesn't pass through that same device it might not suffer the same issue.

I've seen it happen before, unfortunately. And in those cases, when it's something between Consumer A and Provider B that neither of them technically controls, the usual solution I've seen people go for is just to use a gaming VPN to route around the issue. :/