To configure the DIR-850L as a repeater, the device must operate in "Client Mode" (to connect to the main router) and simultaneously in "AP Mode" (to broadcast the extended signal). This configuration is often referred to as a or RelayD setup.
If running a cable isn't an option, consider a dedicated D-Link Range Extender which is specifically designed for wireless repeating and is easier to set up. dir 850l repeater mode