Transmit power is not adjustable on Google/Nest WiFi products. In relative terms, the transmit power is quite low to start with, though. Reducing the transmit power can have counterintuitive effects as well. For example, reducing the transmit power may require sending data at a lower speed, meaning the transmitter may actually be on longer than it would be at the higher power. Since this product is intended for typical consumer use, it's unlikely they will expose this as a configurable parameter due to the increased likelihood that it will result in poor performance and unexpected behavior.