You probably have routing issues, e.g. the same subnet 192.168.1.x used in several routers while you connect via wan ports or something like that.
As you have a wired cable from the main router to the new location, the easiest approach is the "dumb AP" config is where you dumb down the new OpenWrt router. You set the routers to be in the same subnet and connect the cable to a Lan port also in the OpenWrt router.
You need to tailor that advice to the addresses used by your main router, but the concept is quite generic.