The coefficients of the nonlinear arx model are stored in the property called "Nonlinearity". In particular, look at m2.Nonlinearity.Parameters. The coefficients stored correspond to a certain equation whose nature depends upon the type of the nonlinearity chosen (tree partition in your case). Type "idprops treepartition", "idprops wavenet" etc for more information on the underlying equations. A couple of comments:
1. The model obtained by 1-step ahead prediction error minimization can be quite different from the one obtained by simulation error minimization. See the "focus" property which lets you toggle between prediction and simulation.
2. NLARX is a nonlinear model. You cannot just convert it into a (linear) transfer function or a state space model. If you want a linear approximation, use the idnlarx/linearize and idnlarx/linapp commands. If you are looking for a way of importing the nonlinear model into Simulink, use the dedicated "Nonlinear ARX" block which is part of System Identification Toolbox blockset.
Some useful links: For insights into the structure of nonlinear models, see:
For setting up the mask parameters for Nonlinear ARX model block in Simulink, see:
The reference page for idnlarx has lots of information too:
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