Plug your computer into LAN Port 1 (yellow/black) on the DIR-850L. Do not plug anything into the blue Internet port.
Click on Settings -> Wireless -> Manual Wireless Connection Setup . Alternatively, look for "Internet Connection Type" or "Operation Mode." On the DIR-850L, it is often hidden inside the "Internet" tab. dir 850l repeater mode
The D-Link DIR-850L is an old router (released ~2013). Its CPU (Realtek RTL8198C) is slow by modern standards. In Repeater Mode, you will likely see latency spikes (slow pings) and a 50-60% drop in throughput. Plug your computer into LAN Port 1 (yellow/black)