Im using an eppler 423 airfoil, 18ft wingspan, 2.27ft root chord to .91ft tip ch

Im using an eppler 423 airfoil, 18ft wingspan, 2.27ft root chord to .91ft tip chord. I understand that its because of viscous errors and the airfoil polars don’t extend to that region but I don’t understand how to fix it, I have extended the range of the polars and they still don’t extend, how do I fix this.