You are dealing only with variables in your widget? This will not work. learn basics of UE networking 
I can’t put it all in an answer its a lot of stuff. In short, the HUD/Widget is client Owned and not replicated to other players. To change a replicated variable it needs to be in a replicated actor and the value needs to be changed in a server RPC because he is owner / authority over it. So you need to put that vars in your pawn/playerController (for example) and add there the serverRPC which you connect to your hud. So after pressing the ready button it goes to your PC (eg) and calls the setReadyState(true) RPC call which on server sets playerIsReadyBool to true.