Hi RChim,
maybe I should explain a bit more: ISO15693/NFC-V is not natively supported by ST25R3911B. It is supported by the subcarrier stream mode feature of the chip. This means that software needs to do much more compared to other protocols. E.g. it needs to create SOF/EOF/CRC/1of4 coding and on the receive side decode SOF/EOF/CRC/Manchester, etc.
This is why just concentrating on register settings is not enough and you either need to dive into low level details of ISO15693 spec or use our RFAL which implements all of this. This is why I was hinting on the activities in stm32duino to support RFAL. I hope this helps.
BR, Ulysses