Transmission will not shift past first gear

First a reference link to the original thread

https://www.2carpros.com/questions/hyundai-elantra-2004-hyundai-elantra-transmission-problems

Kenny said I should start a new thread.

2004 Elantra GT

No codes being shown, I have replaced the input output position sensors and transmission relay; I tried using that gadget Micro Mechanic OBD reader for a phone to get a better read, but it only worked intermittently (fuse is good) but it worked constantly on another car.

I have read through threads, saw the LR solenoid may be at fault, but it will shift into first and reverse, but will not shift to second gear. It tries but slams itself back into first gear.

I am thinking that since the OBD reader worked sometimes, and the car will go into first and reverse but not second, could the computer (ECM) be at fault? Since it controls the LR solenoid?

Thanks for any help!