Denormalise data in Neural Networks
Normalise using your training data statistics. Save the values used (e.g. mean and sd per feature), treating them as part of your model. Once you have used these values to transform input, they become fixed translate/scale factors in the model. 

Use the same values to normalise test data or new inputs as required. You do not need to calculate new normalisation constants for new data. In fact doing so will most likely reduce the effectiveness of your model.

The same principle applies to interpreting output values if you need to scale those into range that your model produces. Scale according to your training data.