I have had this problem with two dsl-AC56U routers (I had the first one replaced). The wireless drops the 5Ghz connections. When you look for available networks, it usually takes a few seconds to become visible and can be connected to. However, after a shortish period of time, the connection drops and the client reverts to the 2.4Ghz connection. I particularly need 5 Ghz for my iPad Pro. When using it's Bluetooth facility at the same time, internet data transfer speeds half. On the 5Ghz...
Wireless dropping 5Ghz band - ASUS AC56U
Wireless dropping 5Ghz band - ASUS AC56U