Similar here with firmware 20308... my RT-AC68U AiMesh RA RSSI defaults to -70 dBm on 5.0 GHz and -55 on 2.4 GHz. And the client switches node better than with AP Mode. It does step through the 2.4 band before settling on the 5.0 band, but who knows what the client is trying to do, and there is no SmartConnect node band steering here in the 68Us.
I don't expect uninterrupted service when moving to a different node with this equipment... just having it settle on the best node/band in less than 30 seconds using wireless backhaul (slower) once I get there is good. I'm using the same SSID for both bands... now I think I will hide my SSIDs to tidy up the neighborhood.
(Edit: Hiding the SSIDs breaks the AiMesh. AND hides the guest SSIDS, which is not true for my older ASUS RT-N66U. So, don't try this at home!)
The distance/obstacles between nodes (band range/overlap) varies for each installation and may require tweaking the RSSI disconnect thresholds.
Centralized management, not having to juggle multiple SSIDs/connections, using spare/old routers as nodes, and not having to buy/upgrade another vendor's entire canned-mesh system is a win for me.
OE