The "Poly54" fit type is specifying a 5th order polynomial in one direction and a 4th order polynomial in the other.
From the looks of things, you have roughly 40 data points. Your model has 20 terms (many of which are high order). I suspect that you're overfitting like crazy, which means that your model can perform very poorly if used to generate a prediction for out of sample data.
Based on the chart that you provided, you can probably cobble together a simple custom equation with a linear model in one direction and a cubic in the other. Something like
Y = b0 + b1*X1 + b2*X2^3