My first aim was to convert my BCH to BTC, if you say it isn't possible, must live with that.
In this case, the only possible way should be to transfer it back to bitcoin.de and sell it regularly, am I right?
Yes. It is not possible to convert them, the BCH transactions are not valid on Bitcoin blockchain.
You need to have an exchange to exchange the BCH to BTC and send it to your BTC wallet. Having compatible addresses =/ same coin.