I use Gizmo often without internet access and it is rock solid. Make sure you know the current server IP, the device is connected to the wireless router, and enter the server address as (server ip):52199 (52199 is the default port in JRiver, can be changed). Example: 192.168.1.101:52199
If you are running windows on the server there are simple batch files you can run to display the current ip, or just run cmd under programs, then ipconfig. There are many other ways to get there as well.