Hi, keep in mind that the GameMode only exists on the server, so the clients can never access it, it just doesn’t exist there. Further the GameInstance and widgets do not replicate.
You can either
(-) From the server send a Multicast to all clients to start the timer. Then do the timer logic completely local on all clients, when the timer of the server comes to zero change the map.
(-) Do the timer logic on the server only, replicate the current time to the clients (mark the variable as replicated).
Keep in mind that in order to use a Multicast or replicate the current time to the clients, you need to do this on an actor that does replicate and exists on all clients, so neither the GameMode nor the GameInstance (you could use the GameState for example).