Hi, and welcome to the Apple (user-to-user) Community!
Re: Cochlear Nucleus App not working
First, if you haven't already, try re-starting your iPhone (deletes temporary files, may update recent. setting changes, re-=establishes connections, and often restores order if something is not working as expected:
Eg: How to: Restart your iPhone - Apple Support
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If you still need help:
You might want to check that your iPhone has the latest available iOS version, iOS 17.5.1.
Eg: look in Settings > General > Software Updates and you will see if there's an update waiting for you to install.
NB; It is wise to back up your data before installing updates.
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More troubleshooting if needed:
If this is a brand new app you have recently downloaded, check you have the correct version for Apple devices.
or
If you have been using the app ok, and now can not:
Check for a firmware or app update with your hearing specialist, or the manufacturer / or their support website.
Eg: For Contacting Cochlear : https://www.cochlear.com/au/en/connect/contact-us
or
For the Smart App compatibility info.
All the best :-)