I am not displayed as premium user in the forums, despite having a premium licence.

There can be two different causes for this issue.

You have purchased a premium licence code from one of our resellers. The forum login happens via email address. This email address can be linked to your licence. The link for this procedure is here: Connect forum email with premium licence

  Second possible cause: 

You have used another email address in the shop than in the forums. Register anew with the email address used during the purchase of your premium licence.