NIGHTHAWK AX6 AX4300 Mobil Devices Connect to 2.4GHZ But Not 5GHZ.

Good morning to all. I have an issue and i am hoping that those of you with more knowledge than I, can help me resolve it.

When i try to connect to the 5Ghz portion of the router on my mobile devices, it connects but then gives me the following message: "NO INTERNET CONNECTION." 
Then when i switch back to the 2.4Ghz band, everything is fine. 

I just updated the firmware this AM, and even after this update i still get the same message. 

Any and all assistance in this matter is greatly appreciated. Many thanks.