I would check if the NFC reader in the phone work in other use cases.
For example if you can read NFC tags using a third party app.
For example either of these:
NFC Tools on the App Store (apple.com)
NFC-Reader on the App Store (apple.com)
If that does not work the NFC would seem to be the issue.
If that works, it's harder to say for sure.
If you can't read any NFC with the apps
Having done that you can try to isolate if this is a software issue or hardware issue by checking if it works after a clean install.
You can try a clean install of your device after a factory reset:
First make sure you have a backup
How to back up your iPhone, iPad, and iPod touch - Apple Support
Then facory reset your device:
Restore your iPhone, iPad, or iPod to factory settings - Apple Support
And set it up without a backup. See if the issue persists.
If it still does not work it is a hardware issue and will need service:
Contact Apple for support and service - Apple Support
If that solved it there was a software issue. Try restoring the backup and see if it still works.
if that made the problem come back the backup is the problem
Good luck!