Do any other network computers have problems with either Fortnite or other online performance and websites?
Also more information is needed: ISP, make and model modem, make and model router, how man network devices: wired and wireless.
How is your computer connected: wired or wireless? Any interim switches, power line adapters, access points etc.?
Have you tried other known working at speed network cables?
Run "tracert" via the Command Prompt. Target Fortnite's website.
Do the same using "pathping".
Do so several times spaced out during your normal online play.
Post the results that are most representative of the packet losses that occur.