Need help linking secondary remote

[GClef]

I bought a used RC1B remote -hoping to use it with my Mavic 2 Pro that uses the same model RC1B remote. But when I plug the iPhone 6S I use with the GO 4 app, the message I get is:
“The current device cannot use DJI GO 4, please check the relevant manual and download the correct app to use.”
So, thinking it needed a firmware update, I connect the used RC1B to the DJI Assistant 2, for Mavic 2, and it shows up as an Enterprise remote.
What do I need to do to get this RC1B Enterprise remote to work with my Mavic 2 Pro as a secondary in dual remote mode?

 

[TTP]

I would be tempted to set RC1B as Primary controller, and if it works, continue from there, ie set your regular controller as secondary controller, if everything is ok, switch around.

 

[GClef]

Unfortunately, I can’t get the GO 4 app to open while connected to the remote. Just that message about downloading another app.