I’ve had my Apex since July, but I’ve consistently struggled to get reliable reads from my iPhone with it. Other devices like an ACR122U, Samsung J5, Pixel 4a etc. all manage fine, even an iPhone SE (2020) manages it reasonably well.
The Apex is installed into my upper forearm (see the wonderful highlighter work in blue).
I have tried using the top edge of the phone, the back edge and front edge, and I have attached a bad diagram of what I’ve found to work best so far. ‘Best’ means I can maybe get 1 in 10 reads. But I can’t place it reliably for those.
For x-series it’s perpendicular to the implant in both the vertical and horizontal orientations…
However for the flex you want the phone to be vertically perpendicular like you’re stabbing the phone into your arm like a knife (top down into your arm) but horizontally you want the phone to be aligned with the flex… though some photos models require a slight offset… so keep experimenting with various horizontal orientations as you try scanning.
Also… this is just a note about the apex line in general. The chip is extremely capable, and it has a lot of features. Phones these days are really trying hard to probe NFC chips they encounter before allowing apps to do anything with them. This translates to a lot of time for Apex chips… I now have a lot of apps on my apex and that means I have to leave my Andro phone over my apex for at least 2 seconds before anything happens… but it does eventually. If I get impatient and move my phone then the read fails and I have to re-tap.