misuse of high gain antennas
I want to kill myself for making a silly mistake. My ASUS router supports dual bands and 802.11ac. However, I use 8 dBi high gain antennas which suit the 2.4 GHz band only. While the signal strength in the garbage 2.4 GHz band is boosted to - 30 dBm, the 5 GHz signal strength is weakened significantly. That is why all my 802.11ac client devices perform poorly. I promise myself not to commit such mistake in future.