How to create different SSIDs for 2.4 and 5GHz.

This is not correct.  When Guest access is enabled (at least on my RBR50 and RBR750), it appears on both the 2.4G and 5G channels. (the same channels as the primary WiFi SSID).

This question has been asked on the forum since at least 2016, and the answer has always been the same:

Orbi does not support assigning different SSIDs to the 2.4G and 5G WiFi channels.

The vast majority of devices which proclaim IN BIG TYPE "2.4G WiFi required" are simply stating the obvious: The device has only a 2.4G radio chip in it.  If there is no 2.4G WiFi, the device cannot function.  This is like saying, "Flashlight requires 2 C cell batteries."  You can have all the AAs, AAAs, 9-volts, car batteries you want.  No C cells equals no flashlight.

99% of the time, I simply plow through the setup and it works.

For the rare occurrence when the manufacturer has written the smartphone app incorrectly, there are two workarounds:

• Turn down the Transmit Power to 25% on the 5G band (under Advanced Settings) and place both the new device and the smart phone far away from the WiFi access point so that they connect to the 2.4G band.  Then, after the setup is complete, move the device to the desired permanent location.
• Turn off the WiFi system. Create a 2.4G WiFi Hot Spot on a cell phone with exactly the same SSID/password as the primary WiFi. Then connect the smartphone with the device app on it to this Hot Spot and go through the setup.  Once the device has connected successfully, it will remember the WiFi credentials.  Then turn off the Hot Spot, turn WiFi back on, and the device will connect properly.

It would help to know exactly which security gizmos these are.  Someone who watches the forum may have personal experience with them.