I just got the new iMac 27", 5K, 2019 model with the 3.6 GHz Intel Core i9.  I got it to replace my old iMac 27", Late 2012, 3.4 GHz Intel Core i7, with which I used my Fireface 800 for years, using a Thunderbolt adapter.
Once I hooked up the Fireface 800 to the new iMac, with the appropriate adapter, suddenly there was no longer sound. although it would recognize it in Settings.  When I look at the TotalMix FX mixer page (latest version) while iTunes is playing, only the AN1/2 in the Software Playback row and the Main fader are active, that is, showing a signal.  But nowhere else. 
Frustrated, I looked here on the RME user page for a fix and also Youtube tutorials on how the mixer is to be routed and found that when I clicked anywhere on the mixer, on any of the faders, NONE would highlight, as they do in the tutorial.  Yet when I click on the spanner, the fader will expand.  They simply won't highlight. Is this indicative of a routing issue?  I don't know.
When I reconnected the Fireface 800 to the old iMac, the sound came through, no problem.  And when I open up the mixer page of TotalMix FX, and click on any of the faders, they immediately highlight, no problem. 
What could I be doing wrong?  Why does it work on the old iMac but not the new one?  I've loaded the latest drivers on both so that's not it.  They both run Mojave so that's not it either.  The power cable works fine given the sound coming through the older computer so that's not it.  The only difference as far as I can tell is the Intel chip is i9 in the new one and i7 in the old one.
Please help.  I would love to use my new fancy iMac.
Thanks