The common advice — place the extender halfway between your router and the dead zone — assumes uniform signal loss. Walls and floors don’t lose signal evenly, so halfway by distance is often wrong by walls.
A better method
Walk from your router toward the dead zone with a phone showing signal strength. Stop at the point where you’re still at two bars, not one. That’s your extender spot, even if it’s two-thirds of the way there rather than half.
The 5GHz trap
Extending on 5GHz only helps if the extender itself has strong 5GHz reception from the router — otherwise you’re relaying a weak 5GHz signal and making things worse. If your two-bar spot only gets a strong 2.4GHz signal, use that band for the backhaul even though it’s slower.