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If desire this to be done automatically, you will need to purchase a router with those functions, set it up behind the ATT gateway and turn off the gateway wifi.
IF are willing to manually do this, set up the 2.4Ghz guest network, link your children devices to the guest netowrk.
Change the original password on the gateway.
Then when desire to activate or deactivate guest network you will need to access the gateway settings from a connected device (have to be home) 192.168.1.254 move to the wifi page and enable or disable guest network.
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