Authorise DRM removal

Hello,

I bought a book via bol.com that is meant for the Kobo reader. On their website they advise using Adobe Digital Editions to convert the file to .epub and remove authorisation so that the file can be converted to MOBI for kindles (see here: https://www.bol.com/nl/klantenservice/a/5671985870798848/lezen-op-je-e-reader-geen-kobo#particles_block_3). However, when I follow these steps the DRM is still there. What steps can I take to convert the file? 

Thank you in advance.