5ghz AC doesn't go through my plaster walls now. I've got two walls between me and my Airport Extreme and about 10 physical feet and have half signal showing on my phone.
Airport is a very crappy 5ghz ap to begin with so is the Google elcheapo-de-slink.
As I always request in these threads, I'd like someone to point me to a peer-reviewed paper that shows 5GHz being attenuated more by building materials than 2.4GHz.
The usual culprit is that people put their 5GHz networks on the old "low power channels", which nearly every router I've ever seen chooses by default. Half the power, half the signal in the next room, of course.
Choose channel 149 which is full-power.