You have a wireless router in the office but it does not reach into the next room ? Your best bet is to find a way to run a ethernet cable thought he wall and put a wireless router running in AP mode in the second room. A ethernet cable is the best way but you likely can use the powerline network devices to do the same function. They also make powerline devices that contain a AP so you could use a single device assuming a power outlet is located correctly. I would not use the powerline devices if there is any option of use a actual ethernet cable.