Orbi RBR50 router How to split network

NG doesn't officially support SSID separation. 

However you can try this to get your 2.4Ghz device connected:

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.

Be aware that there seems to be a time limit with some iOS or Android devices that will connect to the 2.4Ghz radio for a short period of time before changing automatically to the 5Ghz SSID, even though it's been hidden. The SSID is hidden only, thus the signal is still present. So you might want to also change the power output of the 5Ghz radio to 25%. Then connect your mobile device at a distance from the RBR to ensure the mobile phone keeps connected to the 2.4Ghz radio. After you get the IoT device connected, re-configure the system back to where it was.