...NOT your transmission but your reception. Get a dual antenna USB type and use that. I've used this one at 100' away in similar setup as you and gotten regular over 25 Mbps and at times close to 50.
It also works "out of box" in Linux if you ever want to use that system too.
First off, this might not be what everyone considers advanced, but I've tried everything I know and everything I've read online and I'm out of options. I'm temporarily staying in a detached building outside of a house. The house has fiber and the only wired connection is on the opposite side of the house, so I'm about 100 feet away. I purchased an upgraded router, netgear ac 1200, and an ac750 extender that I placed right under the window inside the house about 20 feet from me. It gets a good connection to the router, and I have a good connection to the extender, and we're paying for 50mbps speeds. Speedtest shows great speeds on the wired computer in the house, and great speeds if you're in the house connected to the main router MOST of the time (can be spotty and slow at times). The issue is that even though I'm getting 60% connect to the main router and 92% connection to the extender when I'm in the outside building speedtest constantly shows me at speeds under 5mbps, very rarely getting up to 10, regardless of which I connect to. It also constantly disconnects. I've tried the tin foil trick on the router to try and direct the signal better my way. I've turned off 5ghz on both devices because I've read it can increase performance. I've tried all recommended channels, and channel 1 is completely free and only being used by me, so there shouldn't be interference. It doesn't make sense to me that I can be on a channel by myself, getting up to 92% connection to the extender, the extender showing 100% connection to the router, but having such ridiculous wifi issues. Any assistance is appreciated. I've called my company and they can't offer additional assistance. They've came out and looked and messed with their equipment but say everything of theirs is working. It has to be a wifi issue since the only wired hookup works great, but I'm not sure where the issue is coming from or what I should do to fix it.