Hey! So, if you want to subscribe to an instance that nobody on your instance has subscribed to before, what you need to do is the following:
1 Grab the full URL (for example, https://feddit.de/c/fragfeddit)
2 Go to the Communities page
3 Search for that URL
4 Switch all search options to "All" (even "Communities")
5 Wait 5-10 seconds for your instance to pull in the community
6 Click through and subscribe!
Now all posts from that community will be pulled in from now on. I know this is a little cumbersome, hopefully the discoverability and subscribing to new communities gets easier after devs have time to work on it post-WebHook removal.