Need a way to enable access to interconnected PCs one their own network. Only the main PC is on the Internet

We have some installations where an acting "Server PC", (Win 10) is connected to the Internet with ConnectWise Control on it, but connected to that "Server PC" are a few "Client PCs" through a network switch without access to the Internet. We need a way to login to those "Client PCs" via the "Server PC" that has ConnectWise Control on it. We tried using RDP but unfortunately that logs you into a different session and not the session that a user is currently logged in.