FWIW, I had issues with FF14 disconnecting until I started using a VPN. Well, I was having even bigger issues with another game and all the technical advice never suggested a VPN but I thought "why not try it, I've tried everything else." And it worked. And in FF14 my disconnects went away too. So, I guess my ISP must be messing with my connections?

Anyways, might not be related to your issue at all. Probably isn't, even. But I'd suggest trying it just to rule it out (There's a free VPN out there but I can't remember the name). And other things that might seem unlikely, because you never know.