ORBI - How do I connect a device that only supports 2.4Ghz?

I just went through a similar setup, but I had no issues.

I have a Genie garage door opener with wifi.  I installed its iOS app.  Followed the steps to reset the wifi connection on the garage door opener.  The Genie starts up by broadcasting its own wifi which is obviously going to be 2.4 Ghz.  Start its app.  It will prompt you to connect to the garage door opener wifi and continue with the setup.  It will then ask you to enter in your network name and credentials.  So long as you have a network that broadcasts in 2.4 Ghz, like the Orbi does, you should be good.  During the setup, it will eventually stop broadcasting its initial wifi network and connect to yours.  I just continued to follow the steps (registration, address, name of my first born, bla bla bla) and it connected just fine.

As a side note, I thought having a wifi garage door opener was kind of lame.  However, I have had those times in the past where I have driven away and forgot to close the garage door.  It is nice to have that alert that informs you the door is closed.  It also comes in handy if you forget your keys - of course I have never done that (ahem) - and you can open the door via the app.

So long as you have a Orbi router/satellite within range of your garage door opener, you should be good.

Edit:

Depending on how you have your security setup, access control, etc, you may need to unblock/allow the new device (your garage door opener) on your network.