Default [Q] Wifi Issues

Hi all, read a number of threads, but none seem to have what I'm looking for. We have two S4's at our house and an Asus RT-AC66u router. My wife's connects to both the 2.4 and 5ghz channels without any issues whatsoever and gets fantastic throughput (>350Mbps). Mine...not so much. It will not connect to the 5Ghz channel at all and has a number of issues staying connected to the 2.4Ghz channel. Any suggestions as to how to remedy my situation? Been happening since Android 4.3 and did not get fixed with the latest 4.4.2 update, my ROM is a stock one. I'm not rooted.

Thanks in advance.
PresidentsDad (aka Marc)
Current Phone - AT&T Galaxy S4 (on stock)
Current Tablet - Samsung Galaxy Note 10.1 2014 Edition
Former AT&T Galaxy S II SkyRocket - with several different ROMs
Former Tablet - Asus Transformer Pad Infinity TF-700 (giant POS)