1. Prevent Lumion from connecting to Internet (Firewall is the best method as you'll need Lumion to stay this way for ever with this method)
2. Launch Lumion
3. Launch the generator
4. Click on the bottom right button in the generator and paste the Lumion code in it (Lumion code is the code showing in the first box of the Lumion activation windows)
5. Save the txt file as the generator ask you to do
6. Open the txt file and copy everything from the second line (included) until the last line before "SerialNumber" is written
7. Paste this in the activation box of Lumion and the benchmark then the program will launch

I've just rendered a scene and everything works fine for now.

But there are two caveats with this method :
You will need to force close the program in order to exit it (with the task manager)
You will always need to paste the code at every start of the software (the benchmark run only once)