I personally could not read my Apex Flex on my iPhone until I put a repeater sticker on it (placement is like this). Yubico Authenticator works for me on iOS, but only on older versions. The changes introduced in 1.11 cause the app to crash when reading an Apex and a java test card I have. 1.11.1 fixed the issue for the java test card but not the Apex so not sure what’s going on there. I’ve just been careful to not update the app on my phone.
Yea, basically this. Fidesmo isn’t a great experience on iOS, most NFC things aren’t 
Also, yubico auth on Android still works with my Apex on the most recent version so that’s something.