It might be due to how old your phone is. The posts about leaving iOS9 behind were years ago!
There was definitely a post about older phones, but I can’t find it right now
For iOS : We support iOS 10 and above for the Current Account. You can use iOS 9 on prepaid for the moment, but we’ll be shuttering the prepaid programme within the next few months.
That was from Jan 2018
Found the post I was thinking of, you’re a few years ahead of this guy!