NEON FAULT 2002 TRANSMISSION

Hi, I have a 2002 Neon automatic. My problem started with an issue I had with the radiator (transmission oil was leaking into it). The problem was fixed, and I drove it for three days until I checked the radiator oil level and it was empty. So I added oil, and that's when the problem started. The gears slip or don't engage until after a few seconds. Then it seems to get stuck in second gear and only goes up to about 40 mph. It won't rev any higher. What can I do?
Change the oil, or...?