How to fix uid in NFC
Since the ID changes peer-to-peer for each connection, you will need to define an NDEF entry with a text value. Make this a fixed ID. On the other hand, on the shield, you will need to read this entry to find the fixed value / ID.
https://github.com/adafruit/Adafruit_NFCShield_I2C/tree/master/examples contains some examples of how to manage NDEF records, but only on the classic MiFare. If you are using Ultralights or other tags, this will be different.
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