Messed up the set up

I started to set my new phone up and got taken away from quick set up. I looked up how to get back on and followed the instructions but didn’t realize the steps were meant for my old phone so now my new phone thinks I’m switching to another new phone and my old phone says it’s ready to be switched and now I can’t do the switch at all! What do I do??! The only choice I seem to have is to erase my phones! I can’t do that to both of them! There is a certain app that I have to have on my old phone in order to transfer over to my new phone. HELP!