I had the same issue; in my case If your router is compatible (gigabit ready, wireless 802.11n or ac) you can set it to use only «n » or “a/c” mode in Wireless Network Mode and on WPA AES. Also WMM Support should be enabled.
As soon as I set the router to the above settings I was getting much closer to 120 mbps depending on the number of devices connecting on this 5ghz signal.
I had trouble using my 5ghz signal using ac only (dd wrt firmwarev24sp2) despite being able to connect to it the previous day. Could no longer connect to it with the settings as described in previous post. I finally got best results using setting n/a mixed 80 MHz channel width, extension channel upper/ lower.
now consistently getting 120mbps down and 20 or more mbps up.