Ping time when there are no network issues is mostly a measure of distance. The light takes time to pass through the fiber.
When it is just some servers affected it is likely some network problem in the path between you and the server. You would have to dig around and see if you can find news of some network issue. A couple years back major fibers were cut by a ship anchor near egypt and a lot of servers were down or running on much slower backup connections. Took them almost 6 months to get that fixed. It can be many things sometime it is the governments, some countries (china for example) place firewalls between their network and the internet. This can greatly slow things down at times.
Bottom line is you can do nothing directly about a problem that outside your house. Only option would be to get a different ISP that maybe has a different path. Some VPN services may also have a different path. It all depends on what the issue actually is and that information tends to not be easy to find.
How are testing the ping spikes with a actual ping command or with something in the game.
If it is with a ping command there is nothing you can really do. If for example you ping 188.8.131.52 and x.x.x.x game server all the traffic still flows your pc----your router---isp router--- internet/other ISP. If there were problems in the first parts all traffic would get high ping times for all sites.
There is nothing you can change on your equipment since it all works fine on your equipment. Your ISP likely is the same. They can't fix stuff that is on another ISP or maybe even in the game servers.
Maybe if you are lucky it is something with the game itself. If you have actual different delays there are tests you can run to try to find where the delay is but lets say you found out it was in some city xxxx using a router from the ISP level3. What can you possibly do about that send level3 email? Your email would be laughed off as another stupid fortnite kid if it was not auto discarded.
It is highly unlikely it is your ISP it is much more likely it is the ISP the game company uses for those particular servers. Different server can use different ISP. If it was your ISP all game servers would have problems.
You would have to contact the ISP that is having the problems.