You can buy software that can convert the HHD format from APFS to other formats. I am assuming the USB drive is a different format than the HHD drive. This will be a cheaper option than buying a NAS. You will still have the same problem buying an ethernet port case for a HHD because of the format. Here are my details based on AirPort Extreme. When I got to edit the AirPort Extreme, I select the following below.
menu:
Base Station-This will have your AE name and password. check the box that says "Remember this password in my keychain"
Internet: Connect Using: Select "Static", IPv4 Address is the AE IP address given by Asus router, Subnet mask (255.XXX.XXX.XX) and router address (192.168.X.X) should be the same as Asus info. all other fields are blank. (XX are for demonstration)
Wireless Mode: select "Extend a wireless network", Wireless Network name will be your network wifi name, Security: Select WPA2 Personal, wireless password, select "remember this password in keychain
Network Mode: Select Off(Bridge Mode), everything else is blank.
Disks: select enable file sharing, Secure Shared disk: select "With a disk password, set password for disk, select "Remember this password in my keychain"
I hope this helps.