Enable Debug and logs and enable 5Ghz. Connect some cellphone and laptop and the minute it gets disconnected and stops working, then note the exact time, lets say the router stopped working at 11:35 am, then search logs in that time frame and look for the logs below and provide the logs to Support.
In my case, I provided, this screenshot " kern
kernel: 1 wl1: wlc_dotxstatus: status indicates too many retries before failure"
if you still get errors in the logs even after updating the firmware, then it means the router needs replacement. They would replace it with newer hardware and repair the old and at some point someone would get it
