The optical TAN method is supported by Outbank. Please check if this has already been activated by your bank for your account.
Then refresh your credentials:
'Settings' > 'Select credential' > 'Update configuration'
The desired TAN method can be selected at the end of the payment.
The individual adjustment of the graphics screen on the iPhone / iPad to the size of the TAN generator is possible with the two-finger zoom.
Where you get a suitable TAN generator, your bank or savings bank can tell you.