It is not possible to use a USB storage device connected to the FRITZ!Box with Time Machine.

USB and Thunderbolt storage devices connected directly to the Mac, the integrated hard drive of an Apple Time Capsule, drives of other Macs in the home network, and dedicated NAS systems that support Time Machine via SMB are suitable for use with Time Machine.

Note:You can use a network cable to connect NAS systems that support Time Machine via SMB to the FRITZ!Box and then use them on all devices in the home network. Refer to the manufacturer for information on whether a NAS system supports Time Machine, for example consult the manual.