Here’s the problem.
Replicated objects are managed by the server, not by the client, Grid is not a replicated variable, so it is locally referenced. The server has access to it’s own Grid, but when the client makes the call for the server, it will look at the replicated object and see that Grid is none (because it only exists on the local client).
Billy Bramer made an amazing tutorial for networking in blueprints, it greatly explains how UE4 handles RPC.
Hope it helps 