I kept entering my passport ID and putting in valid dates but it seemed like the app didn’t want to accept the ID number - As it seemed to keep going back to the ID field and I was unable to see/click the submit/activate button.

The issue I found was that it was just that the keyboard wouldn’t minimise. There was no issue with my data or the app. I couldn’t get rid of the keyboard in the way I do normally (e.g. I couldn’t swipe it down and there’s no way to minimise it using the keyboard itself). However if you click anywhere on the main page outside of the keyboard - It will go away. Then you can see and press the ‘activate’ button.

Simple solution but it’s been driving me crazy and seems like some other iPhone users Apple forums have had some similar issues. I was due to travel soon and was having a panic that I wouldn’t be able to activate my pass in time - So thought I’d post here in case anyone else came across the same issue!