Orbi 850 how to add 2.4ghz device?

You are correct.  The original Orbi ac products had an option to enable/disable broadcasting the SSID on both the 2.4G WiFi and 5G WiFi channels (and still do!)  This option was helpful in those rare cases when devices were difficult to connect.  The newer AX product never offered that option.

My experience has been that the vast majority of Internet of Things (IoT) devices that SCREAM OUT, "MUST have 2.4G WiFi" will connect just fine provided that the user just opens the smartphone app and follows the directions, ignoring all that 2.4G paranoia.  I have connected over a dozen brands of smart plugs, cameras, and other gizmos and had only one that required a workaround.

The gist of the problem is that the smartphone app is poorly written  Either the app discovers that the phone is connected to the house WiFi at 5G and obstinately refuses to proceed, or the app tries to push the MAC address of the 5G WiFi channel to the gizmo when what it should be doing is simply providing the SSID and password.

What is this device?

For those devices that cannot be made to connect, there is another workaround: Use a cell phone Hot Spot.

• Temporarily power off the home WiFi system.
• On one smartphone, set up a WiFi Hot Spot.  (By default, most cell phone Hot Spots are 2.4G WiFi.)
• Give this Hot Spot exactly the same WiFi SSID/password as the home WiFi system.
• On a second smartphone, open the WiFi settings and connect to the Hot Spot.
• Open the smartphone app for the device and follow the directions to set up the device.
• Once the device is connected (to the Hot Spot) and working:
  • Turn off the Hot Spot.
  • Power off the device.
  • Power the home WiFi system up again.
  • Power on the device.  It will connect to the first 2.4G WiFi system that it knows the SSID/password for.
  • On the smartphone running the app, reconnect to the home WiFi system.

This sounds complicated, but will take less than 5 minutes.