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See -> Use tones and ringtones with your iPhone, iPad, or iPod touch

Use tones and ringtones with your iPhone or iPad - Apple Support

• Try to redownload the ringtone you have purchased.

See this part -> Change your ringtone

1. Go to Settings > Sounds & Haptics.
2. Under Sounds and Haptic Patterns, tap the sound that you want to change.

3.Tap a ringtone or alert tone to hear it and set it as the new sound.

You didn't buy a ringtone but a song, is that correct?

To make and/or use a song as a ringtone, you must first make that song suitable for a ringtone.

See this Apple article:  Create and manage ringtones

Create and manage ringtones - Apple Community

Get more help and assistant from Apple directly, see Contact Apple for support and service -> Contact Apple for support and service – Apple Support (UK) 

(the same link as above) See this part -> Buy ringtones from the iTunes Store

1. Open the iTunes Store app.
2. Tap  More.
3. Tap Tones. 
4. Find a ringtone that you want to buy, then tap the price.
5. Choose an option to automatically set the ringtone. Or tap Done to decide later.
6. You might need to enter your Apple ID password to complete your purchase.