I was facing the same issue and could not enable Time Machine on my Asus AX88U and using my Macbook Pro 2019 running Latest Catalina 10.14.4
Setup SMB on your router
Setup Time Machine and select the drive (the drive can be in any format that Asus router reads)
On the Mac Setup a AFP connect to the SNB (afp://192.168.50.1 in the connect window). I also set it up to auto login at reboot. (
http://www.mackungfu.org/HowtouseAFPtoconnecttoaNASonmacOSSierra)
On the Mac goto the Time Machine and try to connect, you should see the AFP drive to backup.
The below link also helped, just replace smb with afp in the link
https://dongknows.com/how-to-turn-your-wi-fi-router-into-a-time-capsule/