This command fails:
root@OpenWrt:~# iwconfig ath0 txpower 20dBm Error for wireless request "Set Tx Power" (8B26) : SET failed on device ath0 ; Invalid argument.
I get the same result if I do it on wifi0.
I know I shouldn't use this to get a better signal, but my radio is rated for between 24 and 28 dB, so I thought it might actually work. I'm going to confirm that that's OK to do with the radio manufacturer (ubiquiti).