When I moved into a new house in 2019 I had a similar really annoying issue - I could keep a single character logged in no problem, but a 2nd character would go R0 at exactly 10min after logging in, out of nowhere.
It's some sort of deduplication that the ISP does that prunes the connection - FFXI is... weird.
I never found any way to solve it by tweaking FFXI settings, 8 months later I moved into another house and the new ISP did not have this issue.
Mudfish has a pay per traffic plan VPU that is perfect for FFXI as it consumes ever so little. It's specific for games, has great routing to Japan (works great for asian mmorpgs in general) and can be applied per-process, so it'll route FFXI but not youtube, not even by accident. Strongly recommend. I learned that it existed from Proth.