Hello,
Today I changed channel to AUTO on my 5GHz network card. After this, interface goes down and stayed unavailable.
After change to static channel interface goes back UP.
A bit more info would be helpful to render any sort of assistance, e.g. which device and TOS version this happening with and perhaps the country code that is set for WLAN.
Usually WLAN events are reported in the syslog, which is accessible via LuCI or ssh.
This could be expected if the country regulates that a frequency scan is to be done. In such case the 5GHz network becomes unavailable for the duration of the scan, can be a few minutes.
That is not expected but can only be looked into if additional information would be made available.