No need to apologize, in our tutorial we use an application in which a Modbus TCP/IP device needs to communicate with a Modbus RTU device as an example. However, this approach can be adapted to nearly any situation in which a Modbus TCP/IP client needs to access another device.
In your case, we need to route communication through the SVR since the network conditions are different. It is not possible to use the pass-through feature in the way that you have described because the pass-through feature is only for use in applications where the PC will pass-through the HMI in order to connect to an external device such as a PLC. The solution in this case would be to establish a Modbus gateway that allows a client on the 192.168.8.Y network to communicate with a PLC that has an IP of 192.168.150.3. The Modbus server driver is necessary.