The external drive used to store Time Machine backups must be formatted with HFS+, also known as Mac OS Extended (Journaled). If the external drive is formatted with APFS, it cannot be used for storing Time Machine backups.
The internal drive in your Mac used to contain the macOS operating system can be formatted with either APFS or HFS+. Both choices will support backing that computer up with Time Machine.
If your computer was automatically converted to APFS with the upgrade to High Sierra this has nothing to do in itself with your Time Machine problem. Such a system is still supported to be backed up with Time Machine.
If you want to reverse the automatic conversion from HFS+ to APFS, you'll need to backup the system, reformat the computer with HFS+ and then restore from the backup (or reinstall a new macOS operating system if you haven't got any data to save). Unfortunately there's no automatic conversion program from APFS to HFS+.
Note that such a conversion from APFS to HFS+ does not in itself improve your Time Machine situation.