If you're having problems with the BoI app on your iPhone, ring BoI customer support and ask what version of iOS is required to run their app.
Check your version of iOS by going from Home Screen>Settings and Software Version will display the version of iOS installed on your iPhone.
The current version number is 14.6, hopefully the version BoI have based their app on.
If your iPhone has an earlier (lower-numbered) release of the software installed, do the following:
1. Back up your iPhone
2. Follow these instructions to backup to iCloud, Mac or Windoze.
https://support.apple.com/en-ie/guide/iphone/iph3ecf67d29/ios
3. Delete the BoI app from your iPhone by following these instructions
https://support.apple.com/en-ie/HT207618
4 Ensure you have your security code
AND SIM unlock code to hand before you start the update process.
4. Update your iPhone (to iOS 14.6) by following these instructions
https://support.apple.com/ios/update. Make sure your iPhone is charged and remains connected to the charger during the update.
5. Install the BoI app following their instructions
6. Follow these instructions to backup to iCloud, Mac or Windoze.
https://support.apple.com/en-ie/guide/iphone/iph3ecf67d29/ios
The instructions are the same for your iPad.
From next year on you won't need different versions of software on your Mac, iMac or MacBook. The devices with M1 or M2 processors will run the same software.
It's a mystery to me and other why BoI can't build a browser version of their account access software. Other banks have managed. Maybe they still use Windoze 95 and Internet Explorer internally and don't see the need to move on.