DIAGNOSIS The 5GHz radio is shutting down. This is typically caused by an overheating router or an automatic channel selection conflict with a nearby radar system (DFS).
FIX Log into the router web interface. Go to Advanced > Wireless > Wireless Settings. Ensure both the 2.4GHz and 5GHz networks are enabled. Check the 5GHz 'Enable Wireless Router Radio' box is ticked. Scroll down. Manually set the 5GHz Channel to a non-DFS channel: 36, 40, 44, 48, 149, 153, 157, 161, or 165. Set Channel Width to 80 MHz. Click Save. The 5GHz SSID should reappear.
IF THAT DIDN'T WORK The router may be overheating. Touch the top of the unit. If it is very hot to the touch, relocate it. Ensure it is upright, in an open space, not in an enclosed cabinet or on top of other electronics. Power cycle the router to let it cool.
HEADS UP The AX21 uses DFS channels (52-144) by default to find less congested airspace. If it detects radar, it is legally required to vacate the channel, which can cause the network to drop. Manually selecting a non-DFS channel prevents this. Ensure the firmware is updated, as updates improve DFS handling.