Factory reset was performed after F/W upgrade.  To confirm the isolated connection, using its default SSID with an 802.11ac device connected, both the windows tool and device tool indicate less than 150Mbps connection.   Difference with your  recommendation, I fixed the channel to 5 and WPA2/AES encryption.  As reported in my problem, only enabling auto channel width would enable also 300Mbps.  Anyway, if the configuration prevents it, I appreciate for your recommendation.  More power to you and this site's contributors