Back up your IPhone with time capsule
Can I back up my iPhone with time capsule to external hard drive
Question marked as
No, the iPhone can only back up to your Mac or to iCloud.
However, if you back up your iPhone to your Mac.....and.....you back up the Mac to the Time Capsule using Time Machine, then the iPhone will be backing up indirectly to the Time Capsule, so you will accomplish your goal of having a backup of the iPhone on the Time Capsule.