If you route everything internally correctly that external connection would work.
But you still need the external gear to be working right and to set the levels in and out properly.
Hit mix button
adjust input channels to that mix with faders
send that result out to f/x via cable out
select layer with channel inputs
send result from your outboard f/x to new channel input
send that to LR or whatever place you plan to use the adjusted sound
You do need to bring up the mix sends for mix 1, Mix 1 should best to post fade for better control sending to an FX unit so the send level tracks with level changes to the main mix.
I would use one of the stereo line input channels to return the FX back into the mixer, ST1 or ST2. Set up the PCM92 for mono in and stereo out.
You could use the AES input on the PCM92 and in the set up IO menu on the mixer set the AES output to be mix 1.