This type of ISP issue comes and goes, it's a matter of connectivity between the different networks and and operators. It can be fine one day and bad the next. It can be fine at a specific time of the day and bad at another. It's completely random.
It's just what the internet is, a maze of cables, routers, operators etc. A broken cable/router usually isn't a big deal as there are almost always alternative routes (but those are generally much slower) but at times they simply "forget" to reroute the traffic... which can make things like this happen. And then there's of course issues that have nothing to do with hardware but purely to do with money (if say operator Y didn't pay the bills to use the cables of X). Good example is how the French operator Free is constantly arguing with Google to the point where Free users get an insanely degraded service with anything hosted by Google (such as Youtube).
This is especially true for routes that see little traffic (I think we can agree AoC doesn't generate much traffic nowadays
). Two options then : your ISP fixes it or you use a VPN (which generally costs money for a good service, like 6-10€ a month...)
There can be issues on FC side too of course (and the operator they use, when I say ISP, I don't necessarily mean yours) but if you play from Europe chances are the issues are not on FC's side since your packets go through so many cables and operators before reaching AoC servers.