Thank you for explaining how WiFi works. My main experience has been running RJ45 Ethernet so I am not as familiar with all of this. I have tried walking around the house watching NetSpot. Yes, the Lyra Voice has no 2.4 GHz radio/signal, all three others do show a 2.4 GHz signal...
If your Lyra Voice is under warranty, I would open a ticket with Asus. Does rebooting the Lyra Voice correct the 2.4 GHz radio? If not, you might try to change it to be an access point or router and see if the 2.4 GHz re-enables in those modes.
How's it going with yours? (This is your thread I apologize for the thread jack).