That’s kind of the problem, because there is no mechanism that lets you tell the Desec servers that they are meant to be secondaries (and the provider* of this service apparently thinks that’s not how Desec is intended to be used). You can’t configure Desec servers to act as primaries to external secondaries either, so they behave as the only servers, and that means they can change the serial at any time without giving notice to anyone. That means you can’t reliably configure your server to be a secondary, and neither can your server be a primary to a Desec server that doesn’t want to be secondary.
*) See the topic I referenced earlier. Nils is one of the people behind Desec.