I use both 2.4 and 5 GHz wifi. All works well, until I connect a USB disk to the router. Then all devices which were connected to 2.4 GHz loose the signal! Those, which support 5 GHz, reconnect to this frequency even the signal is low. Those, which don't support 5GHz, get disconnected. 

When I unplug the USB disk all devices restore the 2.4 GHz connection. Any idea? I'm afraid I'll have to return the router to the store if solution is not found 🙁