Hi Olof,
The Rt-N56u does not include a repeater mode but includes a WDS mode. WDS mode is under advance setting - wireless. Settings will be similar to the link below.
http://support.asus.com/Troubleshooting/detail.aspx?SLanguage=en&p=11&m=RT-N16&s=2&hashedid=WAa6AQFncrceRBEo&os=&no=750