There is no code for trimix. You put the correct firmware on, then you put the trimix patch on is my understanding.
The code for apnea is apparently some kind of math based on the unit serial number, so I don't think there is one code. I may be wrong though. One person had it, but didn't share the code for his.
Nobody cares about the apnea function except me, so unless someone else helps, we are probably as far as we can go on that.
The only thing I can think in your case is 1) you are on the wrong firmware, 2) there might be a way that you can go to the last firmware that takes trimix off and locks it there, or 3) you got a corrupted trimix patch.