On most Android devices you can tap Settings > Connections > WiFi. Then select the network you want to use from the list of available networks .
Read moreHow do I change my Android phone to 5GHz WiFi?
Click on the Advanced option. After that, click on the frequency band. Here you can choose 5GHz or 2.4GHz . And now, after refreshing your WiFi connection, you should be connected to 5GHz WiFi.
Read moreHow do I change my WiFi frequency?
The frequency band is changed directly on the router:
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