I'm not sure if this help you, you may want to try this there is no harm in trying (NOTE: 20Mhz on 2.4Ghz because I own quite a lot of Apple Devices and I own quite a few newer iPad Air/Mini 2 and iPhone 5S which connect to 5Ghz so not a big problem for me)
ASUS RT-AC66U F/W: 374.45
No I thought the same thing I left it on Auto for a while and been checking my AC device and it's still says I'm connected to 5Ghz AC mode even if I disconnect and reconnect I'm always connecting to AC mode [emoji4]
It might be the word is missing or it has been merge into Auto setting...
For some reason this firmware has better Wifi range than .44.... Hey I'm not complaining also my 5Ghz band has been fixed thanks! No issue so far
ASUS RT-AC66U F/W: 374.45
After 5 day of uses on AC66U I didn't have a single problem the only problem I have is the wifi signal which is not good enough compare to .43 and .42and my 5Ghz will only work in Auto mode not in N+AC.
If you looking to extend wifi best option is to get the same brand as your router.. I would suggest another router and put it in AP or simply buy the white Asus adaptor RP-N53 very easy to set-up.. I got 2 in my house
Router upstairs
RP N53 at the front of the house and RP N53 at the back of...
If I was in your position right now and you have to fix the router everyday I would send the router back and get a full refund and buy another router somewhere else mate.
I understand new firmware fixes things and security improvement. Firmware 376.42 and higher should most certainly have a stable security for now and for those who uses ipv6 should try out 376.44 as there has been some major improvement by Asus.
(It's up to you weather you want to upgrade)
Basically long story short every time Asus releases a new firmware you are going to take your time and not work on It instantly... You deserve some time off mate after everything you have done for us :)