New Viessmann AC Stuck at 18 Degrees: A Cooling Failure

I bought a new Viessman brand air conditioner and it only cools at 18 degrees—it does not cool at any of the other levels. It is probably faulty when it left the factory, which makes me question their quality control process. Customer service does not care at all about this product. Although I have written to them many times, they reply with a standard e-mail. The e-mail response is always as follows: the problem you have conveyed has been transferred to our relevant unit. I cannot receive any response from the brand other than this standard e-mail. It feels like they are deliberately ignoring the issue instead of offering real support, and I regret choosing this brand for my home.