There is a update that is due..
But not till the Q1 of 2018..
It is due to Broadcom fixing the Krack issue..
Which ASUS does not have any controller over,
Unsure why, should be simple enough.
There is also a update firmware that was given to me by a forum member.. unsure of it myself.
GT-AC5300_9.0.0.4_382_18724 was given to a forum member recently!.
https://www.asuswebstorage.com/navigate/s/10EE2C959844405692BF375677A60D15Y(Trying to get confirmation from RMerlin to see if it is legitimate as ASUS seem to not want to bring this out to the wild. If it is legitimate.