You have been disconnected (WOW51900118) is an error that happens to me often when I pvp in World of Warcraft. Although there is no official solution from Blizzard regarding this, there are some steps that you can take to fix it.
Reboot the Router
Rebooting the router clears up the data in it and reset the device to its normal state.
Flush DNS
Flushing the DNS will clear any IP addresses or DNS records from your computer’s cache. On Windows, you can easily flush DNS by opening the Command Prompt app (CMD) as an administrator and running the “ipconfig /flushdns” command. Alternatively, you can reboot the computer.
Delete Cache
Open your World of Warcraft game folder and delete the content of the “Cache” folder. For retail WoW, you can find it under “World of Warcraft > _retail > Cache”.
Delete Addons
Deleting all of your World of Warcraft addons will rule out any interference from an incompatible addon. For retail WoW, you can find the AddOns folder under “World of Warcraft > _retail > Interface > AddOns”.
One of these steps should fix the WOW51900118 error and let you log back in to the game.