Hello,
From what I understand you want the rpi to connect to your iphone through wifi and use the phone’s cellular connection to access the Internet.
In order to do so, you need your rpi to act as a regular wifi client and your phone as an access point (not the contrary).
Just set up your iphone as an access point following this https://support.apple.com/guide/iphone/share-your-internet-connection-iph45447ca6/ios
Then connect your rpi to this wifi network using the ssid and password disaplyed on your phone.