I got an Xbox one X from my parents for Christmas and sometimes I have a 200ping at Fortnite for 2 seconds and then 10 again or there's a playful, simply unknown error. Actually have good internet.
Restart the modem
I suspect you are connected to the box via Wi-Fi?
This is always the worst variant, since too many factors can be disruptive. I recommend a LAN cable.
In addition, what does good internet mean? Also important, who or what is using your line at the same time? Last but not least, this can also be a server-side issue of the game.
I'm connected to Wi-Fi 5m from the router and we have a 100k VDSL line.
