Sophos AP 55: It only broadcast 20Mbps

Hi Sophos Community

Good Day

Here is the scenario

Sophos XG210 Fware 16.05 GA

   - Our ISP Provider gives us 100Mbps Bandwidth
            * On the LAN side under Sophos XG Firewall, speedtest results is around 90Mbps which is good :)
            * BUT under Sophos AP 55, speedtest register only less than 20Mbps.

   -  AP is directly connected to Sophos XG Firewall

  I also checked these parameters

• Wireless protection not enabled
           * Enabled

• Wireless Protection registration from the APs zone is not allowed. 
           * Allowed

• AP cannot obtain an IP address from DHCP.
           * Obtained IP

• The Sophos Firewall is not the APs default gateway.
           * Sophos is the default GW of computer's wifi

• Traffic between the AP and firewall is blocked or filtered.
           * No traffic was blocked 

• AP firmware update failed or other hardware problem.
            * Updated Firmware