It's reserved for backhaul on ac68u, from my understanding. Btw on ac68u I see the 5GHz-1 ssid of ac5300 but I cannot connect as ac68u is requesting me the password but failed authentication eventi if the password is correct. So on AiMesh mode only the 2,4GHz ssid is available for clientsThat doesn't correlate either. You should have both bands working on the 68U node.
OE