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If you are not able to detect the 5GHz band network, then you may try to look for the 5GHz button and press it.
Check the light indicators on the modem/router if there's a 2.4GHz and 5GHz band light indicators.
If the 5GHz band is still not appearing after pressing the button, then you may try to reboot the device.
If it fails, then you need to access the modem interface to check the status of the 5GHz band network.
Let us know how it goes.
BasilDV