on android, a number of ways to do that.
one example is my howto guide
Turn your android phone into a media server
i think the big issue, can you run rclone on the iphone itself or not?
that will determine your possible solutions.
Rclone on iSH for iOS
Rclone on iPhone!
i have a big fan of that service.
what i do is rent the cheapest vm from hetzner in the same region as the storagebox.
then can run rclone mount on the vm, using the files from storagebox.
access the files secruly using tailscale.
another option, run a webdav or sftp server on the vm, pointing to the files in storagebox.
from the iphone, access that webdav|sftp server