Although ThinRDP requires no installation on the remote desktops, you might need to enable RDP access if it is turned off.
Once the remote desktop is ready to receive RDP connections and you have set the port and authentication level in ThinRDP, you should be able to access it internally by typing into a web browser:
After accepting the certificate and informing the credentials you will see ThinRDP's main web interface:
This means that ThinRDP is running and you can use it within the LAN.