The transmission control module. There’s a place I think in Hialeah, FLA has one somewhere around $250-300, just install it and go. But is the speedometer working, if not that coutbe the problem cause control module doesn’t know your speed. I helped a lady friend a few yrs ago, hers stopped shifting out of first gear. I checked everything. Installed the new module. That was it. Not sure if you have access to a snap on scanner but should be able to use it to tell transmission to shift, if you can then it’s the module. My opinion.

You sure it’s not in limp mode? We’d probably need some codes and data to even come close to a good guess here.