Diagnosis
A bad firmware update can sometimes corrupt the wireless driver or reset a band-specific setting. The 5GHz radio is probably disabled or stuck.
How to Fix It
Let's try a soft reset to reload the firmware. This won't erase your settings. 1) Unplug the Alien router. 2) Press and hold the button on the bottom of the unit (next to the reset pinhole). 3) While holding that button, plug the power back in. 4) Keep holding the button for about 10-15 seconds until the light ring flashes blue. Then let go. The router will reboot. This often clears up post-update glitches. Check if your 5GHz network name reappears.
If That Didn't Work
You may need to check the wireless settings. In the AmpliFi app, go to Settings > Wireless. Make sure the 5GHz network is enabled and broadcasting. Sometimes updates can uncheck a box or change the channel to one that's invalid in your region, causing it to not broadcast.
Heads Up
Before doing any more updates in the future, it's a good habit to take a backup of your configuration in the AmpliFi app (Settings > General > Export Configuration). That way you can roll back if something goes sideways.