Samsung Galaxy Note 3 Keeps Disconnecting Wifi

On AT&T service.

Solution: Thanks to android central forum (Telespielstube):

(Option 1, didn't quite work for me)

for all those who having WIFI issues constantly disconnecting. I have found this on xda-developers.com (source below).
"Go to phone
On the keypad dial *#0011#
Automatically should go onto a screen
Press menu
Go WiFi
There should be a button there saying WiFi saving and it should be ON turn it OFF
And then reboot
Done "

But even if it doesn't, this is a new thing for me. Like a secret access.

Option 2: Config wifi on phone with static router ip.

Option 3:

Valerina: 1 Power the device off (remove and re-insert the battery).
Valerina: 2 Press and hold the Volume Up key and the Home key, then press and hold the Power key.
Valerina: 3 When the device vibrates; release only the Power key.
Valerina: 4 When the Android system recovery menu displays; release the Volume Up and the Home key.
Valerina: 5 Navigate with the Volume keys to the Wipe cache partition option and press the Power key to select.
Valerina: 6 press the Power key to reboot system now.

I tried all 3 above. And also set Auto Connect to AT&T Hotspot in Advanced Settings of WIFI on phone. Didn't work.

Then I upgraded the firmware of my ASUS RT-AC66U router, which on the new firmware info page did mention something about fixing 5ghz issues. Which appeared to have fixed the problem:
ASUS RT-AC66U Firmware version 3.0.0.4.376.3626
All region can use this version.
[Important] Please press the reset button more than five seconds to reset the router after firmware updated

Fixed the 5GHz N+AC mode UI issue.