I had 2 ASUS RT AC67U working perfectly using ethernet with a dead area between the two. Purchased an ASUS RT AC68U but it would not join the other routers. All three have Firmware Version:3.0.0.4.384_45149. Just tried the suggestion above "pressed WPS button while powering on and release 15s later" and it worked straight away. Thank you for the suggestion. I now have an excellent 3 node AiMesh system.