I’ve noticed multiple times where the 5GHz channel which I primarily use to connect to the AR750S seems to disappear occasionally where the SSID no longer seems to be visible. Disabling/re-enabling the 5Ghz interface through the web admin panel seems to fix things but that’s annoying to have to do every time.
Are you using a DFS-impacted channel? If so, there is typically at least a minute before the driver selects the channel before it is permitted to start transmitting.