https://archive.is/KQNcc without paywall.

It’s tricky. The DLC also was promised pre-launch and even already sold with the premium edition.

This question was posted in the community of their client (liftoff).

Liftoff allows searching on other instances than the one you are currently signed into. But if you open a community through that search, you can not subscribe.

If you search through the community you are signed in, it works as expected.