Irregardless of what TP-Link claims for Time Machine backup support, here are some clarifications. The following is true for an AX6000.

1. Router will not recognize drives formatted to Apple APFS.

2. Router will recognize drives formatted to Mac OS Extended but will not allow TM backups (backups fail). This is true with any variations such as encrypted or not. I also tried the suggestions from a post to turn off journaling and that did not work either.

3. Router will only recognize drives formatted to ExFAT. Note that when using this for TM backup, TM becomes a resource HOG. My internet connections slowed to a crawl, sometimes taking several minutes to load a site. The backup eventually completed, about 3-4 hours. I tried to do another backup, it looks for the backup disk, then that changes to Preparing Backup  for a long time and then it just quits without making the backup.

As an FYI: I spent several hours with Apple upper level support when the drive was formatted APFS, and we concluded that TP-Link router cannot recognize this format. Unfortunately TP-Link makes no mention of this, they merely say to plug in a USB drive. After the call, I began experimenting and that's how I discovered 1-3 above.

As a side note, I also discovered that formatting a drive using Apple disk utility can vary the hierarchy results, depending on whether the drive is formatted from the utility directly, or as I discovered, using the disk utility in Recovery mode yields different results when trying to use as TM backup on the router. In one scenario I got the error could not create backup disk. in the other scenario, I would get the error that I do not have permission to make changes (tried to add my name under Sharing and Permissions and it would not let me).

So all this tells me that TM Backup is not truly supported on this router in spite of the fact that it is listed on one of the FAQs as a router that supports TM.