I think you are confusing it with adding aimesh nodes. At least at one point it worked a bit more reliable with stock firmware. I don't think it's the case now. Aimesh works fine for me. Just trying to keep my RT-AC86U up to date on Merlin.
I have 2 RT-AC86U aimesh nodes connected via ethernet to RT-AX86U.
Trying to GUI upgrade both RT-AC86U to a latest Merlin (3004.386.13_2) firmware from 3004.386.12_6.
One of the units upgraded just fine, but in other upgrade keeps failing. i tried different browsers, powercycling between...
I have 2x RT-AC68U running latest Merlin firmware (384.7_2). One is setup as router, and the other as AP. They are connected with 1Gb ethernet link (LAN1 to LAN1).
I also have 2 wired clients and about a dozen various wireless clients.
When I look at View Clients screen, both devices display...