From the sounds of it you are on Fire to the Premises and have the NTD (NBN box) in the guest room?
The modem will still need to be physically plugged into that box via ethernet. You can buy a long ethernet cable (you can get them via jaycar and I would assume places like officeworks would sell them) and move the modem. If you're looking at either moving the NTD itself, or setting up some ethernet wall plugs and running cable through the walls, you can contact telstra in regards to this via 1800 TFIBRE (1800 834 273). It will come at a cost to you however