Lemmy is still in a very early stage and development is accelerating super fast in the last weeks. I read somebody say Reddit was also very hard to use 15 years ago. However I can absolutely understand the frustration that can come with Lemmy and all the young apps.

I'm no expert but I see two possible solutions for your problem.

One could be that your instance does not know the community yet. All communities that are local on your instance should be found by Jerboa. The most other big ones too, because other users of your instance have already joined one of them (only after 1 person subscribed, the community is known for everyone). If you are looking for niche communities, you may have to search with the whole link, so search for https://feddit.de/c/ich_iel instead of ich_iel and wait for 10 seconds.

The other one could be to to set standard links for jerboa. You can do this on Android in the app settings of Jerboa, I don't know how the option is called in English but it should be easy to find. Same place where you delete storage and cache, handle permissions etc. If you press on "add links" you can select all the different instances (feddit.de, Lemmy.world etc.). Maybe you can open the community you want to locate by opening the link to it with Jerboa.

That's only some ideas. You can also consider trying out other apps such as Connect for Lemmy or Liftoff. It seems Slide and Boost will also be developed if I'm correct. It needs some time until everything is round, but so far most of us are amazed what has already been done!