Vitra Concealed Cistern Leakage and Lack of Support

I am deeply disappointed with the Vitra concealed cistern that we purchased, despite its high cost. It started leaking after only two years, which broke our trust in the brand. We paid a service fee to have the gasket replaced, but the leak came back within a week. When I called the service again, they said they couldn't find the Vitra gasket. Even though their products come with a 20-year warranty, they don't stand behind them. Every time they come, they try to get out of taking responsibility by making us pay to fix the problem. I want Vitra to be responsible for the problem I'm having. If my problem isn't fixed, I'll bring it to the attention of the Consumer Arbitration Board.