When you say it doesn't fit, it does mount, doesn't it? Your N2020 will have the same mount as the D40. Does it rotate and click when you try to mount it?
Assuming it does, there are two things to try:
clean the contacts on both the lens and the camera (with a dry cloth, or if they're really dirty with a pencil eraser, taking care to not get bits inside your camera)
make sure the lens aperture is locked at the minimum (f/32 I think)
I don't know which model of the 70-210 you have, but my guess it the camera won't meter, so you'll have to use manual mode.