How do I apply a hill-curve or s-curve to this plot?

I want to plot a curve to this plot, it contains nan values an 0 values, i have tried 'poly3' fit but I am still not happy with the result. The output should level at 13500 max and start at 0. I do not know how to find the coefficients for more complicated plots.

I know of lsqcurve() but i do not know how to calculate the bo term

[AgonW]

Poly3 fit

[AgonW]

for using lsqcurvefit() i get the following error

lsqcurvefit stopped because it exceeded the function evaluation limit,

options.MaxFunctionEvaluations = 2.000000e+02.

is there any way to overcome this?