Here's the official, straight-from-the-project procedure for installing OpenWRT on my new router:

Install a newer version of the vendor firmware on the router. Not any version, not the newest version; one specific version from more than a year ago.
Install DD-WRT on the router. That's not a typo; not OpenWRT: DD-WRT. Not one specific version, but it must be whatever the latest beta release is, and you must get the one for your country-code-version of the router.
Install an older version of the vendor firmware on the router. Older than the one that was originally on the router. Not any older version, a specific one from more than a year ago.
Install OpenWRT on the router. Not, mind you, LEDE, which doesn't support my router. But OpenWRT does. Even though the projects have merged again ... sort of.
Also, go to some guy's web site and download a WiFi chipset firmware blob, SSH it to the router, then install in within OpenWRT. But only if you want the WiFi 5GHz to work, of course.

Open source!