The ability to specify the upper and lower bounds for coefficients in the NLINFIT function is not available in the Statistics Toolbox.
To work around this issue one can consider using the LSQCURVEFIT function in the Optimization Toolbox. The LSQCURVEFIT does not offer the exact same capabilities, for example: LSQCURVEFIT does not allow using a 'Weight function' like NLINFIT as an input, however, one can manually customize the objective function to include this information.
