Upgrade iCloud storage

Hi everyone

I was hoping someone could advise about a loop I’m stuck in trying to upgrade my iCloud storage (iPhone XR, payment details all up to date, Uk based)

I purchase by putting in apple password

this takes me through to iTunes app and a pop up window to go to my banking app to authenticate the purchase

I authenticate in my banking app but when go back to iTunes the pop up window (which I need to tap ‘I have authorised’ on) has gone. I’ve tried so many times now I think my bank will flag it as suspicious activity!

many thanks in advance for any advice.