If you set the derviative equal to zero you will get the constant solutions to the equation. They are

and

This divides the plane into three parts.

If you pick an N value less than zero lets call it and test it in the ODE you get

This is negative so the derivative is always negative in this regrion

If you pick an N value

So the product is positive this gives that the derivative is always positive here

If you check the last region you will find the derivative is always negative

The horizontal line is unstable all solutions move away from it.

The horizontal line is stable all solutions approch it as time increases.

Try to sketch the above it will help.