Answer
ST60 doesn't support [HT40] 11n mode in 2.4G.
In OpenWRT, the PHY is set by adding (Logic OR) the two settings of Band-1(2.4G) and Band-2(5G).
So, [HT20] and [HT40] two modes are always set.
Need to modify the OpenWRT source so that only [HT20] is set in case of 2.4G operation of the ST60 modem.