How close to the router is your iPad or iPhone when the dropouts occur?
With the 386 release from ASUS, it looks like the region on the router is not changing from the default of US which has the maximum power available to be less on the lower 5Ghz channels.
If you SSH to your router and issue the following command, it will tell you the max power for each available channel.
wl -i eth2 chanspec_txpwr_max
Output:
36 (0xd024) 18.00(dbm)
40 (0xd028) 18.00(dbm)
44 (0xd02c) 18.00(dbm)
48 (0xd030) 18.00(dbm)
149 (0xd095) 25.50(dbm)
153 (0xd099) 25.50(dbm)
157 (0xd09d) 25.50(dbm)
161 (0xd0a1) 25.50(dbm)
165 (0xd0a5) 25.50(dbm)
The following command will tell you the region it is currently on:
wl -i eth2 country
By default it is on:
US (US/0) UNITED STATES
If I issue the following to change it to Australia:
wl -i eth2 country AU
I now get the following but it resets back to US if I restart the router.
36 (0xd024) 21.00(dbm)
40 (0xd028) 21.00(dbm)
44 (0xd02c) 21.00(dbm)
48 (0xd030) 21.00(dbm)
52 (0xd034) 21.00(dbm)
56 (0xd038) 21.00(dbm)
60 (0xd03c) 21.00(dbm)
64 (0xd040) 21.00(dbm)
100 (0xd064) 21.00(dbm)
104 (0xd068) 21.00(dbm)
108 (0xd06c) 21.00(dbm)
112 (0xd070) 21.00(dbm)
116 (0xd074) 21.00(dbm)
132 (0xd084) 21.00(dbm)
136 (0xd088) 21.00(dbm)
140 (0xd08c) 21.00(dbm)
144 (0xd090) 21.00(dbm)
149 (0xd095) 21.00(dbm)
153 (0xd099) 21.00(dbm)
157 (0xd09d) 21.00(dbm)
161 (0xd0a1) 21.00(dbm)
165 (0xd0a5) 21.00(dbm)