With the setup described and JRemote2 connected to the MC Server, you should be able to play to the Client by selecting the Client PC name plus Zone to play to in JRemote2.
i.e. When you tap the Zone icon in the bottom left corner of JRemote2 (the target icon), you should see a popup dialogue that allows you to select the Zone to play which will be named as;
{MC Client Computer Name} if you want to play to the default "Player" Zone on the MC Client.
OR
{MC Client Computer Name}: {Zone Name on MC Client}
JRemote will then display the Zone being played to as;
{MC Server Computer Name}: {MC Client Computer Name}
OR
{MC Server Computer Name}: {MC Client Computer Name}: {Zone Name on MC Client}
If tapping the Zone icon in JRemote2 doesn't display the options above, you have a DLNA problem, such that DLNA devices aren't being seen on your LAN. I have that issue currently, as I have a cruddy ISP router that seems to block visibility. JRiver BingoSSDP makes the router see DLNA devices correctly, as does running PingTools, both run on my Android phone.
Of course, you need to have Media Network turned on in the MC Client for the above to work. But you need that for a MC Client/Server setup to work anyway. As Jim mentions, the MC Client needs to have "DLNA Renderer" turned on under "Options > Media Network > Advanced", but you probably already have that selected.
Alternately you can connect JRemote2 to the MC Client, and select a Zone from the MC Client to play to.
EDIT after your second post:
The MC Server only needs to be a DLNA Server. The MC Client only needs to be a DLNA Renderer. JRemote2 will act as a DLNA Controller.
Well, in theory. But I tend to have my MC Server and MC Client set to run all three, DLNA Server, Renderer, and Controller, just to allow flexibility, controlling and playing from anywhere to anywhere. It can take a little while for DLNA components to be found on a LAN, which I sort of speed up or fix by running BingoSSDP or PingTools, which span SSDP (DLAN Discovery) packets all around the LAN, which lets the router know they exist.