Google sets the wifi country code to whatever is in your sim card. If the country does not have a corresponding list of enabled 5GHz frequencies, then a lot of these frequencies get disabled. This mod simply sets the country code to US. There is no GUI or anything, just enable and restart and you're all set. Android marshmallow only. I tested it on my S7 edge and it's working fine.
Edit: Added OREO 8.1 version. Testing working on Pixel 2 XL.