I have used readers like this before and I do find them finicky. Issues with recognizing file names, and also with power requirements. I had to try many combinations of readers and cards to find one that worked every now and then, and then always make sure I used a very specific file naming syntax. I gave up and switched to a Bluetooth device to read cards.
Back in the day they used to work much better, but then Apple made a change in iOS that lowered the output voltage. After that, many readers and other devices (keyboards, microphones, etc) would no longer work. Some readers still work if they and their card draw low enough. Other readers need an external power supply.