There are many different AActors that can be owned by the client, not just a APawn. In your case, if the client is truly connected and past login, then he has an APlayerController that you can put a “server reliable” RPC on.
Any actor that is spawned where the APlayerController is part of the owner chain (pawn, hud, etc), can have an RPC on it.
The client can call this function to have work done on the server. Alternatively, based on what you are trying to do, also after login, the server can call a “client reliable” RPC on the same object owned by the client, but on the server, to do work on the client.