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