Amateur working on my own repair here.

Car will go into overdrive fine first time, then when slowing to a stop (engine on) it will no longer shift into overdrive from that point on. If I stop and re-start the car, it will shift in overdrive one time again like before

I deduced that it was the ECU most likely so i got a used one (same make and year), installed and now the car won't turn over. Car started fine with old ECU reinstalled.

I tried the paperclip in the port trick for 30 minutes (youtube trick) to charge/calibrate the used ECU and got flashing lights on the dash for 30 minutes (key on, car not started) while clip was in ports 4 and 13, like was supposed to happen but then car still wouldn't turn over after.

What's my next move and why? Do I need to buy a thingee to plug in to laptop/car and reset ECU? Or is the used ECU bad maybe? Appreciate any advice or help! Thanks