how to detect 125khz tags with mfrc 522
You are correct NFC is 13.56MHz the key chain ones you are referring to are most likely 125kHz The reader you have will only read 13.56MHz this is the NFC and HID cards. If you want to read the Key Chain ones you will need a reader like this one. Weigand 26 Card Reader Most Phones will not read the 125kHz