Firmware choice? on ac3200 and ac87u

I have a ac3200 in 2 floor and ac87u the ground floor,cable from ac3200 to ac87 u.
Using ac87u wireless extender/reapeter mode.
Running latest asus firmware.
My internett speed is 100/20.
Sometimes internet pages is slow,tried speedtest.net and speedtest speed ok.
Im using cable/wired.
So is there anyone who can recommend a stable firmware for these two