Installing OpenWrt-18.06.1 on a Asus RT-AC66UB1

Setup router as per generic instructions.
Can see the existing Asus variant of dd-wrt.
Trying to upload the . . . factory.bin file the response is something like:
said file is not acceptable it is only possible to use official software

Does this mean I can no longer flash openwrt onto this router?
If I can - - - how do I do that?


There is another thread about it here: Invalid Firmware Upload on Asus RT-N66U

Thank you - - - sounds like possible but not terribly straightforward.

I am having a great deal of difficulty finding a router that I can buy that is supported by Openwrt.
Am on a wireless broadband connection so I really don't need major hp but don't want to buy items that are months from eol which seems to be all that gives me availability AND Openwrt.

Any suggestions as to how to find a router 'today' that works (and is in the under $120 usd class)?