Some further reading and experimenting since posting. My Windows laptop "sticks" with 2.4GHz, unless I manually change the laptop to use 5.2GHz with the Intel tools. My wife's Linux laptop automatically went to 5.2GHz.
I have a couple of devices that are 2.4-only, so I actually don't want to turn off or down the 2.4GHz.
Band steering is only supported on Meraki APs. The MX with wireless capability or the Z3 don't have this capability. If you want devices to connect to the 5Ghz, set the transmit power of the 2.4Ghz to the minimum.
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What about smart devices that only connect at 2.4Ghz? I have a fan that only will connect to a 2.4. I have not been able to connect to my new Z3 AP as the device probably see the 2 bands and gets confused. Any solutions to this issue?