OK I discovered that the Digimax was plugged into the optical ADAT output.
Ive now plugged the ADAT output of the Digimax into the optical ADAT ‘in’ socket on the FF.
Now I’m getting signal in TOTAL MIX on ADAT 3/4
How do I get the signal to route to Cubase?

If the signal is not in the cubase VST connections input 3/4 then check input channels 5/6.
The Fireface setting SPDIF to optical can cause such shift.
May you also check the Cubase VST connections for having those input channels enabled.
The Mac CoreAudio driver provides the signals directly to the DAW, no further routing needed, same as under ASIO in Windows.

What do you mean with "hardware playback" ?
In Totalmix there is the top row chanels called hardware inputs, only there should your signal appear unless you do some routing.
The middle row of channels in Totalmix are the software playback channels, signal from Cubase or any software is there.
The third row on the bottom of Totalmix are the hardware outputs, there is signal visible only when it is routed there.

There is probably something not yet ok in Cubase. There are no special specific connections, they are the VST input and output connections, you can give individual names to those channels and enable them for Cubase.