iMac 27" mid-2011 has Limited WIFI Channels 5GHz

My iMac 27" mid-2011 has limited WIFI Channels. Only from 1 to 13. I have seen others who had similar problem, but that doesn't fix my problem. (the one that said he changed Timezone to automatic)

Basically I need to connect to my new Router (5GHz) but to no avail, iMac only shown as follows:

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Interfaces:

en1:

Card Type: AirPort Extreme (0x168C, 0x9A)

Firmware Version: Atheros 9380: 4.0.74.0-P2P

MAC Address: (censored)

Locale: RoW

Country Code: TH

Supported PHY Modes: 802.11 a/b/g/n

Supported Channels: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13

Wake On Wireless: Supported

AirDrop: Supported

Status: Connected

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Please kindly assist. Thank you very much.

Software Versions:

CoreWLAN: 11.0 (1