SO a couple of things,
Turbo will jump in if I don’t say anything.
So to quote him; literally
Basically for simplicity,
RFID = 13.56 MHz High frequency (Including NFC)
RFID = 125 kHz Low Frequency ( Including 134kHz- xBT & xHT {Amal has said 134kHz but DT Website says 125kHZ either way it’s Low Frequency} )
RFID = there are many Others, but these are the MAIN two WE deal with
Now that’s out the way
That is strange!!!
I saw in your post in the other thread you took it out to read.
Now that it is under your skin, and you are effectively a water balloon, there will be a bit of signal loss. So now it is important to find the sweet spot.
I would suggest as you have an iPhone and you know it works ( or at least did ), use your same phone initially and the same App, but make sure you have your phone positioned with the top edge (I believe) perpendicular over the NExT and push down as far as comfortable.
Check out THIS thread for antenna and Phone info…
If this doesnt work, Test with a compatiable Android phone ( same link as above )
Let us know how this goes