Hey @bosstjan,
On the server container, you can only configure the platforms for which tags exist.
If a tag does not exist, but the platform has an API through which you can send the required data and events, you can use the HTTP request tag: Tag Manager Template Gallery.
For RTB it doesn’t look like there is a tag for sGTM and it doesn’t look like they API for events right now. Therefore, it can only work on the client side.