DIAGNOSIS The 5GHz radio has likely crashed or been disabled. A firmware bug or overheating can cause the radio module to shut down temporarily.
FIX Log into the router at routerlogin.net. Go to Advanced > Advanced Setup > Wireless Settings. Verify the 'Enable SSID Broadcast' box is checked for the 5GHz network. If it is, perform a hard reboot. Unplug the router's power, wait 30 seconds, plug it back in. The 5GHz SSID should reappear after the boot cycle completes.
IF THAT DIDN'T WORK The router may have reset its Smart Connect feature, merging bands into one name. In the same Wireless Settings menu, look for 'Smart Connect'. Turn it OFF. You will then see separate settings for 2.4GHz and 5GHz networks. Re-enable the 5GHz SSID manually.
HEADS UP If the 5GHz band drops out repeatedly, the router is likely overheating. Ensure it's upright on a hard, flat surface with at least 4 inches of clearance on all sides. Do not place it on carpet or in a closed cabinet.