Not sure it helps per se, but I have a couple of Windows 10 devices that can't see an SSID on a new ET12 2.4GHz network.
One is a laptop (old dell XPS 13) that dual boots into Mint and same hardware with Mint sees the network fine and connects so it's definitely not the laptop hardware it's the OS.
Looking at a couple of threads on Microsoft site suggest there are known issues there too with Win 10 following updates. So far none of the suggestions I've found have helped me.
Unf I am new to Asus routers, so I'm stumbling through a lot of this as I learn but if they are Windows devices it might be worth checking out the network settings.
Well, fixed the Dell, despite Microsoft and Intel automatic driver checkers telling me it was the latest driver, it wasn't.
Downloaded manually (from the Intel site !??), updated and it's seeing the new network SSIDs now.