I have said this in previous posts - so my apologies if you have already checked this.
I have an AC86U running Merlin (the actual build number is unimportant - appears to happen on all Merlin builds, but that is not to say that it doesn't also happen on vanilla Asus - not running vanilla Asus, I have no repro).
For me, the symptom, is that the 2.4Ghz appears to disappear, when in reality, it is still there, just that when connected to on 2.4GHz, it's as if the router is blocking onward packet transmission and/or the WPA password is incorrect (it isn't).
For me, the only solid fix, is to remove power and then reconnect after about 20secs. In my particular situation, Rebooting has no effect (although I see others recommending it) the ONLY fix (for me) is to pull out the power. Have you tried doing this?
Best wishes,
Woody.