Ah, it’s because nobody on your instance is currently involved in that community, so it hasn’t been federated to Beehaw yet. To fix that, go to the search tool, make sure that you have “All” selected for all four filters, and enter “https://lemmy.world/c/ece”. If you get a community result, you can click on it and subscribe to the community and then crossposting should work. If you get too many results, try switching the scope from “All” to “Local”. If you get no results, it means that federation is pending but delayed. Give it a minute or two and try the search again; eventually the community should show up.

Maybe also crosspost to [email protected], which should be good at helping out with such questions?

Every server just has a cache … there is no profit for the whole network …

I wouldn’t say that caching is no profit. Yesterday there were several times when lemmy.ml was struggling or effectively down for some people, but despite complaints over there I could read lemmy.ml communities just fine through my instance. Caching meant that I was isolated from the service interruption, and the lemmy.ml server was isolated from my contribution to its load.