iOS9 App for New User
One of my friends was so impressed with what YNAB has done that he's signed up for the free trial. One of his first questions was "How do I get the app installed?" which seemed like a simple question since he also has an Apple device like I do, running iOS 9 (9.3.6, to be precise.) The App Store version requires iOS 11, however there must be some version that works on iOS 9.3.6 since I'm currently looking at it. I have an iPad and he has an iPhone, so there could be some difference there.
How can he get an iOS 9 compatible version of YNAB?
Ironically, one of the things he hopes YNAB will help with is saving enough money for a new phone. :-)
If you're unable to upgrade from iOS9, then I assume your iPad is an iPad 4 or older and the phone is an iPhone 5 or older, which means they're running older 32-bit chips from Apple (A6 or earlier). In which case, YNAB isn't going to work with those devices or with iOS9 at all.
iOS 11 and 12 can run on an iPhone 5s or newer and an iPad Air or newer - you don't necessarily need a brand new phone.