I can't find it on the 8, it should be in sounds and notifications... Tap on ringtone and run down the list. If you used a music clip as ringtone, you can just share that between the two units...
I had a similar problem with a notification sound from my Maxx 2 that I liked. I ended up recording to to my laptop. I did some minor edits then saved it to the new device. A bit of effort but it worked.
Ringtones should be in a folder named Ringtones (the upper-case R counts). Copy that to your PC, then copy it from the PC to the S8, to the Ringtones folder.