Sounds very similar to an issue I was having in January. Here's the two things I can recommend you doing:
- Load any other always-online game at the same time and see if it's just XIV or both games. If it's both, it's probably your ISP
- While my ISP fixed their crap when I was having my issues, using
Mudfish VPN was a savior for me. You can set it up only for XIV and it's prepaid, so buying as little as $3 can last you months, depending on use, because it's just filtering XIV and avoiding congested nodes.
Best of luck.