iPhone cannot connect to Wireless-N PLEASE HELP!

Hello, I just bought a new router last week, which is new Linksys which has the Wireless-N Server. Suddenly I notice that my iPhone could not connect with it. Whenever I press, I put in the correct password then the message show up "Unable to join the network..." Normally, my iPhone can connect but since I change to new router, it could not connect anymore...

Thanks for any Help
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Device Info:
iPhone 3GS (Unlocked) - Thailand
FW 3.1.2