@Allan So here is what I have so far. I followed the instructions for Quick Apple Raspberry Pi Machine and formatted a hard drive to HFS+ and installed netatalk on a Raspberry Pi 3b clone; everything worked fine. I went into Time Machine settings and the network drive was recognized and added. Since this was over ethernet, the backup took about 13 hours to complete.

I then connected the drive to my Archer AX50 and configured the settings for a Time Machine shared drive. Time Machine did not see the drive. This seems logical since the router is sharing the drive using SAMBA and not netatalk.

I'm hoping this gives you some clue as to what is going on. Let me know your thoughts.