Hello yoosuf86,
If your external drive is not recognized by your iPhone 15, first ensure you are following these steps from Charge and connect with the USB-C connector on your iPhone 15 (that you've already looked over)
Connect your iPhone to your iPad, Mac, and other computers
You can connect your iPhone to your iPad, Mac, and other computers to transfer data, like your photos and videos, using a USB-C cable compliant with the USB-C standard, such as the cable that comes with your iPhone. These cables are widely available from Apple and other manufacturers. Just connect one end of the included USB-C charge cable to the USB-C connector on your iPhone and the other to the USB-C port on your computer.
Your iPhone will charge while connected to these devices. iPhone 15 Pro and iPhone 15 Pro Max support fast USB 3.2 Gen 2 speeds up to 10Gbit per second when using an optional USB 3 cable.*
*The USB-C cable that comes with your iPhone supports charging and USB 2 speeds. If you want to use a USB 3 device, use a compliant USB 3 cable that supports 10Gbit/s.
If this continues after looking over that, reach out to the manufacturer of this external drive. They'd be the best equipped to look into the compatibility of this device further.
Cheers!