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