1998 Audi A4 SPEEDO SENSOR

Computer problem
1998 Audi A4 Front Wheel Drive Automatic 121000 miles

it used to take a bit of time for the gears to shift. The problem now is that from o-45 miles, the gear changes only once and if it does, it struggles a lot. My mechanic told me it is the speed sensor. He replaced the sensor but the problem was still there. He later told me that the
E.C.U is faulty and not sending the right signal to the gear box therefore the car does not know which speed it is moving at. After he checked the E.C.U, he came back to tell me the instruments cluster(speedo cluster) is faulty that is why I still have the problem. Can an instrument cluster cause the gears not shift properly?