1_Is there a step-by-step tutorial to set up the Geiger and connect it to Radmon?
2_once the Geiger is finished. Is it necessary to have it connected to a computer or is the network cable enough?
I'm not aware of any step-by-step tutorial, at least not on the forum here. There are many snippets in other posts. You may find something on the internet.
Once you are up and running you won't need it connected to your PC, just a power supply and the data will use the ethernet cable and your network/internet.