Maybe what you experience is to be expected. Found following statement in the initial commit to the device:
Wireless: 5GHz is known to have lower RSSI signal, it affects speed and range.
There is a superlong discussion on github about this device and even more discussion in the forum.
Check out Follow on conversation for TP-Link Archer A6 v2 5GHz improvements, including user support - #166 by humanOS
I am not going to read all that 
have fun, if you want to dig deeper.
Last but not least, there also is a pull-request for Archer A6 EU/RU, so if you are not sure, if you actually REALLY have the US version, you could try that one too.