DIAGNOSIS This is classic 5GHz DFS interference. The router is detecting radar signals and switching channels automatically.
FIX Log into the router's web interface. Go to Advanced > Wireless > Wireless Settings. Change the 5GHz Channel to a fixed, non-DFS channel. Choose 36, 40, 44, or 48. Apply the settings.
IF THAT DIDN'T WORK The issue might be a firmware bug. Go to Advanced > System Tools > Firmware Upgrade. Check for a newer version. Update if available. Reboot after.
HEADS UP Using a fixed channel can reduce performance in congested areas. Letting it use DFS channels is usually better if radar isn't present in your location.