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It looks like https://coincodex.com/ will let you convert your STO to BTC. You would then need to send your BTC to https://ionomy.com/ and buy DTC -- as far as I know, Ionomy is the only site that lets you trade BTC.
After you have your DTC, you can send it to your D.Tube username.
Hope that helps! Good luck!

*I mean Ionomy trades DTC!

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