@jerclarke I've been there. It eventually stopped working. I tried everything and eventually gave up. My backups are up to 700GB, took almost 24hrs each time and starting from scratch every week...

@jerclarke "the only reference in the universe to this question" Yup, it's really strange that this isn't a more common issue.

@Kevin_Z Who suggests using NTFS for Time Machine?

The login page of the C2300 (and probably other TP-Link routers as well) does not work well with Safari on MacOS. As you can see in the attached video file the password fill option does not work and...

Have you tried that? Hooking up a drive with Mac OS Extended (Journaled) format to the C2300 router and running a Time Machine backup? Seems odd that the C2300 would be able to read a Mac only drive.

Please answer my original question: “What file system should the USB drive be formatted with to work with Time Machine?”