Not sure how it works in combination with steam mate. I've got the game via EA App.only EA App to offline or when you are in game and servers are connecting?
I have the game on steam and it opens EA App too when starting.
Once I've activated offline mode in the EA App, EA FC 24 automatically doesn't connect to the servers! Maybe you could try to open EA App and activate offline mode, before starting the game via Steam. If that doesn't work, you have to make sure the game doesn't connect to the ea servers in game. (Before loading into the main menu and when playing kickoff it also tries to connect)