WPA2-enterprise on USG20W with Active Directory Auth

I understand from the documentation that apart from Radius, USG20W supports WPA2-Enterprise using Active Directory as authentication service. I configured the following but doesn't seem to work. 1. Object->AAA Server->Active Directory, I have already setup the link to my AD server. Tested and return OK. 2. Object->User/Group, I have linked my AD server created in the last step to the correct Group Identifier. I tested under the configuration validation screen and it showed OK when I enter username belongs to the Group Identifier. 3. Object->Auth. Method, I created a new method called "WirelessAuth" with the Method list included the AAA Server I created in the first step. 4. Under the Edit WLAN settings screen, I selected WPA2-Enterprise, with Authentication Type as "Auth Method", Authentication Method as "WirelessAuth", TTLS Certificate as "default". 5. On the client computer, I selected Microsoft PEAP, Unchecked "Validate server certificate", with Secured password (EAP-MSCHAP v2) as authentication method. Can someone please shed some light? Thank you!