Thank you for all the replies and suggestions. I'm happy to report that after completing @OzarkEdge install notes above, not only is my RT-AC86U repaired, but my AiMesh system is running better than ever!
So one of my Aimesh routers (RT-AC86U) just failed. I still have two working, another RT-AC86U and an RT-AC68R. I've never been particularly happy with Aimesh (signal drops), but I'm still willing to try to make it work if you think it's my best option. I'm also willing to start from scratch...