Hello! My name is*****I am a certified computer professional who will assist you with the problem you described.Most wireless network routers work on 2.4 GHz frequency, but some of them work on dual frequencies - 2.4 GHz and 5 GHz. That way, you can improve the connection quality since you get 2 different wireless networks - one that works on 2.4 GHz frequency and the other that works on 5 GHz frequency. Both your Asus network devices mentioned here support dual frequencies; however, in order to benefit from the 5 GHz connection, you need to have a device that supports 5 GHz connection. Most computers, phones and tablets only support 2.4 GHz connection since its built-in wireless network card works only at that frequency. To be able to connect to a 5 GHz wireless network and utilize the maximum speed, you need to use a 5 GHz wireless network adapter that works on 802.11 AC mode.
Devices such as PS 4, XBOX 360, TiVo support only 2.4 GHz frequency.
For your computer, you can purchase an external USB 802.11 AC wireless network card that works on both 2.4 and 5 GHz frequencies.
For further assistance, let me know.
The 2 clients you see is the Asus RP-AC68U itself - one for 2.4 GHz and the other for 5 GHz. In order for wired devices to be shown in the 5 GHz list, you need to set up the media bridge to connect to the 5 GHz network of your Asus RT-AC66U router. If you have both 2.4 GHz and 5 GHz networks enabled on Asus RP-AC68U, just disable the 2.4 GHz one. You may need to restart both devices for the changes to take effect.
Let me know the results.