Alas my old DSL-AC68U is running RMerlin firmware, so cannot test, but issue maybe the AiMesh Primary router not having latest firmware ? what is Primary RT Model ? are there any other Nodes on AiMesh topology ?
I am still surprised ASUS got the DSL-AC68U to support AiMesh – Node – Ethernet, so will be good if others have managed to get this to work ?
I did notice (whilst I was testing) on my set-up that I can connect via AiMesh (wifi) and then via the Primary AiMesh wifi router change the Interface option to suit an Ethernet connection LAN 1 or LAN 4 for the Node – my set-up is via Wifi currently
P.S are we assisting SNB Forum members again 
(pic below (not my setup) shows ‘INTERFACE’ option on far Right Lower dialogue box)

In a completely sane world, madness is the only freedom (J.G.Ballard).