How do I transfer custom ringtones from my iPhone 5 to my iPhone 6s?

guess they don't want to tell you

Hi RMNA,

Thanks for using Apple Support Communities. Based on what you stated, it sounds like you are missing tones you purchased. I would recommend that you read this article, it may be able to help the issue.

Restore missing tones on your iPhone, iPad, or iPod touch running iOS 8.1.1 or later - Apple Support

1. Open Safari on your iPhone, iPad, or iPod touch.
2. Go to itunes.com/restore-tones.
3. Sign in with your Apple ID.
4. Tap Restore. If your device is eligible, you'll see a message that reads: "This may take a while. You can continue to use this device while we prepare your tones. You will be sent a push notification when they are ready to download."
   If you see, "Your tones were not affected," your tones won't be restored to your current device. You can try the website from your other devices.
5. Tap Done.
6. When you receive the notification on your device, tap Download, or, if you receive the notification on your lock screen, swipe to download.* All of your tones will download, including any that are explicit.
7. Check to make sure that your tones are now on your device:
   Ringtones: Tap Settings > Sounds > Ringtone.
   Alert tones: Tap Settings > Sounds > Text Tone (or any other alert listed).
   If you assigned purchased tones to your contacts, these will also be restored.

If you purchased tones with more than one Apple ID, follow the steps above signing in with each Apple ID at step 3.

You can repeat these steps on your other iOS devices, if needed.

Cheers