Servers for FIFA 18 have largely been decommissioned by EA. Online Ultimate Team features are generally unavailable, though local multiplayer still works perfectly.
If you are looking for general details about the game, here is a quick breakdown of how this specific release was received and handled: fifa 18 rom nintendo switch 2018 top
The legal method to obtain a Switch ROM (.XCI or .NSP format) is to buy a physical copy of FIFA 18 and use a hacked, homebrew-enabled Nintendo Switch to dump your own private backup using tools like NXDumpTool. Servers for FIFA 18 have largely been decommissioned by EA