I had the exact same issues and wracked my brain for close to a month trying to fix it but I DID fix it. The issue is that your ISP is basically cutting you off when it gets to a certain file on the download. The reason for this is long and boring but basically since FFXIV file downloads are hosted through Akamai / APNIC which is P2P, your ISP thinks you're downloading something like a torrent.

In order to get around this, you'll need to use a VPN HOWEVER, and this is what I learned the hard way: MOST free VPN's are going to default to UDP and not TCP virtualization. So go get CyberGhost VPN (free) or TorGuard (paid) and make sure you go into the VPN client and change it from UDP to TCP. If you don't change it from UDP to TCP, your download will continue to fail.

The only ports you need to have open on your firewall are 80 and 443. Just make sure your launcher is closed before you start the VPN client.