Applies to: Focusrite iOS Control, Scarlett 6i6 + 18i8 + 18i20 (2nd Generation), Scarlett (3rd Gen Range), Clarett Range, Red Range
First, try all the troubleshooting steps from our guide below linked below.
If the issue is still present, follow these steps:
Close Focusrite Control On the Mac
Go to the Activity Monitor on the Mac.
You can search for Activity Monitor at the spotlight search located top right of the Mac desktop.

Now quit all Focusrite Control server tasks in Activity Monitor:

Now re open Focusrite Control and launch the Focusrite Control iOS app, it should now see the device.