It is best to start fresh. Press and hold the Reset button on the back for 10 seconds while the unit is powered on.
Place the DIR-868L halfway between your main router and the dead zone. If the repeater is too close to the main router, you don’t extend range. If it is too far, it will have a weak signal to repeat. Look for a location where your phone shows 2 bars of the main router’s Wi-Fi. dir-868l repeater mode
Download the official D-Link DIR-868L manual (Revision C) from D-Link’s support site. It is best to start fresh