To begin with, we wrote an appeal on the main account where we explained everything with the hope that this would be enough. However, Amazon rejected the appeal. Then we decided to start working on reinstatement of the related account. We updated the information in Seller Central so that it matched the information in the documents (Utility bill) as much as possible and sent it to Amazon. The account was immediately reinstated.
The main account was restored after an email with the same explanation + mentioning that the related account was active.