how to turn off 5ghz on Orbi RBK50 to connect a device

Netgear provides a simple mechanism to accomplish this. The Orbi RBR50 has a feature which many users have found allows them to set up devices which require the smartphone to be attached to WiFi at 2.4G:
Connect a device with a large screen to the Orbi web interface, http://orbilogin.net
(computer, laptop, tablet)
The user name is "admin" and the password is the web interface password, not the Orbi 'app' password.
Navigate to the Advanced Tab, Advanced Settings (at the bottom), Wireless Settings.
On the Advanced Wireless Settings page, uncheck the box under Advanced 5G Settings that reads
Enable SSID Broadcast
Click Apply
The router will go through a cycle which takes foreeeeeeeever.
On the smart phone, go into Network settings.
"Forget" the Orbi WiFi.
Search for available WiFi systems.
Select the Orbi WiFi
Enter the password and Connect
The smartphone is now connected to the Orbi at 2.4G
Note that if you attempt to "double check" by opening the Orbi Attached Devices page, that page takes several minutes to update.
Just go with it.
Open the device smartphone app and go through the setup process.
After the device is connected and working, undo the process.
On the web interface, check the box and click Apply
On the smartphone, Forget the Orbi, find it again, and connect again.
This takes less time to do than to read. It has worked for me several times.

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.