Make sure you didn't accidentally establish a radio schedule (Wireless -> Professional tab).
Otherwise, check the System Log page to see any indication as to what could be happening.
Might also be worth resetting back to factory defaults and reconfiguring your router in case you might have some corrupted settings in nvram (especially if you have been upgrading the FW over multiple versions or from a much older version).
Asuswrt-Merlin: Customized firmware for Asus routers
Github: github.com/RMerl - Twitter: RMerlinDev
See the sticky post for more info.