Register the device

Paste the UDID in Fodide when it asks. Fodide registers it with your key. If Apple refuses, do it by hand: Certificates, Identifiers & Profiles → Devices → + → platform iOS, a Device Name, the Device ID (UDID) → Continue → Register.

Up to 100 iPhones per membership year. Removing a device does not free its slot until the year renews.