Orbi RBR10

Netgear provides a simple mechanism to accomplish this:
Log into the Orbi web interface (http://orbilogin.net)
Navigate to the Advanced Tab, Advanced Settings, Wireless Settings
Uncheck the box, "Broadcast SSID on 5G"
Click "Apply"
All devices that are currently connected to the Orbi will remain connected.
On the smartphone, go to wireless settings and "forget" the Orbi SSID
Search for available WiFi networks. Select the Orbi WiFi. Enter the password. and connect.
If you are curious about whether the smartphone is actually connected at 2.4G, you have to wait a few minutes, because the "Attached Devices" display takes a while to update.
Open the app and set up the 2.4G device.
Once the device is connected and working as it should, reverse the steps.
Check the "Broadcast" box and click "Apply"
"Forget" the Orbi WiFi. Search. Enter password. Connect.

IF this is not seen on the 1 series Orbi, you might try changing the power output of the 5Ghz radio to 25%. Turn OFF The RBS. Then connect your mobile device at a distance from the RBR to ensure the mobile phone keeps connected to the 2.4Ghz radio. You may have to walk the device out to a distance to ensure it will connect to the stronger 2.4Ghz swignal After you get the IoT device connected, re-configure the system back to where it was.