@terziyski Do I need to set them both like the main is 192.168.23.18 then what should I put in the TP Link?

@terziyski I've already done that set up, I finished the quick set up but there's still no internet connection.

I'm using a TP Link router to be a extender now I have already set it up, I connected a ethernet cable plugged in LAN on my main router and on WAN on thr TP Link TL-WR840N Router then set it up on...