Apos explicitly include an iOS compatible cable option which you would have seen when you selected your purchase from the drop down menu. Apos could certainly be clearer that an apple certified otg cable is needed rather than optional, but the bigger issue seems to be that your phone manufacturer doesn't provide any documentation at all about this issue:
On iPhone, use files stored on USB storage devices and SD cards.
support.apple.com
Apple have very obviously broken compatibility with widely accepted standards to prevent you from using devices outside their ecosystem. When an OS has been so obviously and deliberately broken it seems a little unfair to blame the peripheral device manufacturer. Bottom line is you just need to buy the cable that you saw was available when you bought your g5 and you'll be good to go.