Copying custom ringtone made years ago from iPhone to iPhone doesn't work

I've have had a custom ringtone made by myself from an MP3 for at least 8 years on at least 5 different iPhones. When changing phones I've always used the feature that copies all data and settings from the old phone to the next, but this time apparently the device management policy by my new employer prevents this. I have tried now for hours to get the custom ringtone to the new phone, but without success.

On the old phone (both phones are iPhone 13 mini) I cannot see the ringtone anywhere, except on the menu where you select the ringtone. It's not in Files, it's not in Downloads or Purchased folders in the iTunes store app and it doesn't show in the Tones folder in iTunes app on my pc. I tried also taking the same steps I did years ago, shortening the MP3 to under 40 seconds and changing the file type to *.m4r, but iTunes refuses to show the file with that format in the Library and refuses to copy any other file format to the Tones folder. I'm getting desperate here. The song I want to use is not available to purchase on the iTunes store either.