Others on the forum have explained that some athletes with severe ADS will be able to maintain nose breathing at very high heart rates. For this reason you cannot rely on nose breathing tests as the sole indicator of effort. If you try and train at the max you can nose breathe your HR will be too high and you will not actually improve your aerobic system and will beat yourself into the ground.
Use HR as the threshold indicator (but know it does change day to day and can improve over time). I would suggest doing a HR drift test to determine your AeT and then spending ~90% of your training in Z2 or Z1.