I have ATT U verse for an ISP and the modem/router they give you is abysmal as far as Wifi signals go. So I got a Netgear R7000 Nighthawk and it doubled and sometimes tripled my DL speed in the furthest location of the house. I have it set in AP mode and it is plugged into my U-verse router and I have disabled the wifi on the U verse router. At least I thought I did as the wifi light is now off on the Uverse tower. However, when trying to access Plex through my Roku I get stutters and buffering and some movies just straight up won't even play. This is perplexing because on the ATT router it actually plays better even though the DL speeds are like half. What gives?
A usual suspect for this is a congested wifi channel. Try using a different channel for your 2.4ghz and 5ghz freqs. Also, if one has poor performance on the 2.4ghz freq, try switching to the 5ghz freq.
You were absolutely right. It turned out that I had not in fact completely shut off the wifi on the U verse router and there was a bunch of crosstalk jumbling up the connection. Thanks for your help.