I do NOT know how this problem occured except that it started years ago.
I just got a smartphone

and downloaded the app but when it came time to change FP+s I could not login with the same information as I use on the website. Thinking I maybe needed to register for the app, I tried that; but could not because an account already exists for my email. So, as a last effort, I tried a different password that I used about 10 years ago - that got me into the app. Then, of course, I could not access my reservations because they are already linked to the web site account with the current (not 10 year old) password.
Somehow parts of disney IT (BOG FP site also uses this old password and I should have worried about this when I discovered that) has old information on me - this is nothing I can change. I have tried emailing them and have 3 different phone numbers in 3 different email answers that link me to different WDW reservations lines, but none of them have been answered even after holding as long as 52minutes23seconds! I really want to sort this out before I get to WDW so I have time to become familiar with the app. Ultimately I will need a tech support number not rezzies or park info anyway - anyone have a tech support number that might help?