as above

Same. There’s another thread on TM, too. https://community.tp-link.com/en/home/forum/topic/824592?sortDir=ASC&page=4

Wow, it occurred to me to search for APFS support on TP and the number of threads about this issue that have been locked without solution for the past 3 years is astounding. If I knew this before I bought TP router I would certainly have stayed away. The number of related issues are just piling up - all having to do with backing up using Time Machine. 
 

The fact that TP doesn't currently support APFS is a big problem, because Time Machine formats drives using APFS and because APFS allows for multiple logical drives without specifying size (total disk size can be dynamically shared, so one can backup multiple Macs to same drive without having to physically divide drive and knowing ahead of time how big each portion is). 
 

If TP supports APFS, at least we'd be able to start a backup on APFS formatted drive by connecting it to Mac and then moving it over to USB of router and have Time Machine continue using it (AFP support for wireless backup continues for now as long as backup had been done on drive before, but new backups cannot be done). 
 

TP has been ignoring this for so long that people buying their products and counting on TP to solve the problem are sadly too optimistic. If you haven't bought their products yet and you need solutions like Time Machine backup over the network to a USB connected drive, look elsewhere. This thread should be enough evidence for you.