Why can't I shift past second gear?

After a minor fender bender that involved replacing the radiator and disconnecting the lines, I had to refill the transmission fluid, after adding 5 quarts it will shift into 1st and 2nd, but will not go any faster than 25 mph, and has to be floored in order to go that fast. I've tried using the manual shifting mode to put it in 3rd but it acts like it's in neutral or still in 2nd. Prolonged testing caused a burning smell from the front bottom of the engine bay.