Later firmware revisions (after 2018) or the DIR-850L REV B may have limited repeater functionality. REV A (US/AU/EU) typically retains full support.
The DIR-850L is a dual-band AC1200 router. When set to , it acts as a wireless range extender—picking up an existing Wi-Fi signal and rebroadcasting it. This is not its primary function, but it’s a built-in feature. dir 850l repeater mode
If your main router supports WPS (Wi-Fi Protected Setup), you can use this, though WPS is known to have security vulnerabilities. Later firmware revisions (after 2018) or the DIR-850L