The code, P0753, deals specifically with the shift control A solenoid in the transmission. There are a few things that can cause it to set this code. It could be the solenoid is disconnected, a damaged wire causing an open circuit in the wiring, or the solenoid itself is bad. Also, based on the code you provided, I don't believe it has anything to do with the valve body.
I have attached two pictures below. They are diagnostic flow charts related to this circuit and to this issue. Take a look through them. You will need to start with number 1 and go step by step. By doing this, it should lead you to the problem.
Here are a few links you may find helpful when you test:
Keep in mind, since you just replaced the transmission, there is a good chance it is simply a poor connection at one of the wiring harnesses. Make sure no pins are damaged in the plugs as well as the plugs are tight. Also, make sure no wires were accidentally pinched when the trans was installed.
I hope this helps. Let me know what you find or if you have other questions.
Images (Click to enlarge)
Friday, April 3rd, 2020 AT 7:10 PM