Strongbox supports YubiKey's only via the following methods on iOS devices:
Please not that YubiKey over USB-C is NOT SUPPORTED because Yubico have decided not to support this configuration.
This is not a Strongbox design decision. This means that on certain iOS devices without NFC or Lightning support (certain iPads e.g iPad Air (4th generation) at the moment) you CANNOT use a YubiKey to protect or unlock your database.
For the list of compatible YubiKey keys please see Yubico's Strongbox product page here: