Hi Craig,
Do you still have the purchased ringtones in question on another device?
I presume you've looked at Use tones and ringtones with your iPhone or iPad - Apple Support. A key detail might be this one:
You may not be able to re-download some previously purchased tones.
If I recall purchased ringtones were originally a one-time deal that you needed to get backed up into iTunes and look after, but iTunes 12.7 removed built in support for tones in iTunes, which might have been when they changed the rules. Personally I've always rolled my own as described in Create and manage ringtones - Apple Community. If you cannot get the purchased tones back you may need to take the same approach.
You might reach out to Official Apple Support in case they can restore the ringtones to your purchase history somehow.
tt2