Because the DIR-850L is no longer a router, you must disable its DHCP server (or let the wizard do it).
Share-ready post: TP‑Link DIR‑850L — How to set up as a Repeater (quick guide) dir 850l repeater mode
It’s always best to start fresh. Hold the reset button for 10 seconds while the device is powered on. Step-by-Step Setup Access the Dashboard: Because the DIR-850L is no longer a router,
– Functional but outdated.