If you can't download the ringtone here:
Settings → Sounds & Haptics → Ringtone → Download All Purchased Tones
Go to https://reportaproblem.apple.com
Sign in and check your purchase overview - and see if the purchase of the ringtone has been registered. If you can't see the ringtone in the list, it's because there was an error when you bought the tone, and therefore you can't download it. However, you have not paid for it either.
If the ringtone is on the purchase list, you can request a refund at the same place. Alternatively, you can contact Apple Support directly and ask where you are not receiving your ringtone.
If it's been a long time since you bought the ringtone, you may not be able to re-download it if it's no longer available in the iTunes store.
You can contact Apple Support here:
Contact Apple for support and service - Apple Support