Wrong gearbox in my Mazda 6
 

Hi!

I bought a Mazda 6 from 2014, diesel. I noticed that the car was vibrating when driving in 6th gear. The seller's mechanic changed some parts in the car, and it did not help, so in the end he said that I will get a new gearbox. He found a new gearbox (probably from a car that was in an accident), and the car stopped vibrating. However, the screen on the panel does cannot see when I shift from 5th gear to 6th gear. So, while I am driving in 6th gear, the computer thinks that I am driving in 5th and suggests me to shift up. The mechanic was very surprised, but admitted, that probably the gearbox was wrong (did not match). Now a long time has passed, and he keeps postponing this work. Then my question is the following - can it be such that Mazda produces the same gearbox but with different computers attached to them? In other words, should I try the solution in which I keep the current gearbox, but change the computer attached to it? While driving the only problem that I have is that the computer cannot see when I shift to 6th gear, but it can see all the first 5 gears. I am asking, because I am tired of this problem, so maybe this would be a good way to fix the problem. I have no idea whether the wrong gearbox affects the car in other ways. Does it?

I would like to hear your comments about the situation - is it bad for the car and how would you solve the problem. Thank you in advance!