ASUS RT-AC3200, Windows 10

I'm having a strange experience with this router. When I attempt to make a new connection at 2.4 ghz I get an access denied response. I have around 30-35 devices connected at the 2.4 and 5 ghz frequencies, and these existing devices will automatically reconnect on a reboot. The 5 ghz channel has not problem accepting new devices. It is the 2.4 ghz frequency that will reject any new device for some unknown reason.

The strange thing is that if I reboot the router, I am able to connect a new device immediately after the reboot router is password protected with WPA2-Personal, however I set up a guest network that was not password protected and any new 2.4 devices failed to connect and were rejected also. Not really sure what is happening or why it is happening. But as I said a new device will connect after a reboot - but then the rejections of new devices starts again.