I had this issue myself starting pretty much with the beginning of Shadowbringers. My issue is my ISP hits a certain node when playing FFXIV and I would just disconnect every couple of minutes or maybe an hour later. I never actually "fixed" it as such, rather I always use a VPN now. I can't really play any game without a VPN but I'll sure take doing that than disconnecting many times. Hopefully one day it'll get resolved.