A while ago I got hold of a couple of DLink DIR-865L AC1750 802.11ac routers as there was a generous cashback offer.
I was originally using it as a glorified access point, but since updating my main router to carve off a variety of ethernet segments for home automation, guest networks, etc, I wanted to expose these as different wifi networks, using the DIR-865L units to extend the wifi network, with ethernet backhaul to the main router.
To get OpenWRT onto the router (where the router's IP is 10.0.1.200 and the tftp server is 10.0.1.16):