I don't believe the RT-AC66U is supported anymore (at least not with AiMesh v2.0).
The RT-AC88U is fully functional with AiMesh.
What the latter is known to trip over is using all 8 LAN ports. I suggest you only use ports 1-4 and ignore ports 5-8 on all Asus 8 port routers if you want a stable and glitch-free network. If you do need more ports, a $10 or $20, 5 or 8 Port switch is the way.
Be sure you have the latest firmware installed on the RT-AC88U.
And don't forget you can use the RT-AC66U in AP mode (wired backhaul) too.