Check each channel (1, 6, and 11 and 13 if applicable in your country for the 2.4GHz band and every single channel on the 5GHz band for best throughput (not signal strength) and lowest latency).
RMerlin firmware or one of his forks (john9527 or hggomes) is highly recommended over Asus stock firmware. Many bug fixes, more features and stability too.
In addition to what was offered in the link in post number 2, you can try the following.
Place the router at least 10 feet / 3 metres about ground level (not just the 'floor').
Place router with at least 3 feet / 1 metre clearance above, behind, the sides and if possible, below too.
Place the router as centered as possible in the area that is to be covered (when viewed in 3D).
Adjust not only the orientation of the router (by 30 degrees or less and testing at multiple locations each time), but also the antennae placement too. For a single level area to be covered, an antennae orientation like '|||' maybe optimal. For a multi level area, '\|/' may be better. But this is not written in stone, experimentation is required.
Also consider not placing the router within 3 feet / 1 metre of a device that is frequently used wirelessly that close. I have found that maximum throughput is achieve between about 8 and 12 feet of the router to the closest device, with most being fastest past 10 feet.