My Transmission?

We have a 91 automatic Honda Accord with only 110,000 mi. One day it was driving just fine the next day when started up we smelled what was like an electrical burning smell and we could not shift it out of park We did the emergency unlock and was able to drive in rear, 1st and 2nd but it did not want to take off at 3 and 4th gears the Honda dealership wasn’t quite sure what it was, but figured it was the transmission A new transmission was put in but the exact same problem was still there It did not want to go in 3 and 4th gears Then they said it was ALSO the computer. How is it that even after the new transmission was in place it still needed the computer part and they can still say the transmission was bad. Why could it not just have been the computer. How do I know it was both.