Remember that each Actor in multiplayer game has Client side and Server side. All Widgets and input events are called on the Client side. If you want to send a variable to the Server, you need to call an RPC event in a replicated Actor (switch on the “Replicates” option in Actor class defaults) and send the variable via this event. If your variable is already on the Server side, you can set this variable to “Replicated”. This will update this variable for all Clients.
Here you can read more about UE Networking.