The App work fine on my iPhone 15 Plus an 15 Pro Max.
Have you tried a soft reboot on the iPhone not just turn on and off? (Volume up, volume down and then power button until the Apple) Are you able to log into MyBT via the web? And have you deleted the App and re-downloaded ?
So did the reboot. Reloaded the mybt app. I can log into my account via the website ok. App still not working. Any other ideas?