Not Square-Enix support, but... out of curiosity, have you tried a gaming VPN? ([URL=https://mudfish.net/]MUDFISH[/URL is a good one, and cheap/borderline free.) If the issue is somewhere between your network and the datacenter, trying a gaming VPN will potentially route around it. If a gaming VPN still dies on you, then at least you've ruled out the pipeline between the two ends and know the issue is at one or the other.

Alternatively, if the computer's portable, taking it to another location—a friend's house, whatever—and trying to connect there will help narrow down if it's your network or FFXIV's end.

Basically, trying to tweak variables until you can narrow it down to where in the overall network path the issue might be.

If none of that works, the open source network logging tool Wireshark runs on macOS just fine, and I suspect some of us here can help talk you through using it to try to see if any additional data can be gleaned from it.