I've been using an 86U for the main AiMesh router, and a 68U as the AiMesh node. I set it up once about 10 days ago, and it's been running smoothly ever since. However, I have little insight into how it all works, unfortunately. Both the main router and the node are managed through the UI on the main router, which of course includes Smart Connect, which was absent on the 68U when it was not used in the mesh setup. It's using wireless backhaul, and I have no idea which band is being used for that function. I suspect the 2.4ghz because of the distance between the boxes, but I don't know for sure.
All clients -- PCs, streaming devices, IOS devices, whatever -- have been working just great with Smart Connect enabled. Clients connect on both bands through the node, which I can see on the Network Map status page. Other than that, I have little idea how it performs its magic. I'm not obsessed with connection speed, as I don't do much copying between devices on the LAN, and my ISP connection is just 40/10, which is more than matched by the mesh system on either band.
Based on the many horror stories on this forum about people having grave difficulties getting AiMesh to work, often when they have somewhat complex configurations, I'm a little reluctant to make any mods to the setup.