That's probably because in your instance no one has manually joined any community on lemmy.ml, and since no one has joined the local server has not created a "local" or federated version of that community. Still, you can do it manually but you will need to login to your Lemmy account in a browser, no matter if it is on desktop or mobile.

Now, the "manual" method sucks and I just know it because I was one of the first to join Lemmy when the Reddit drama happened, but that's the way it is:

Suppose I am a user of lemmy.dbzer0.com I want to join the community c/kde in the instance lemmy.kde.social, to do that I need to adjust the URL of my community in a special way, which is like this:

https://lemmy.dbzer0.com/c/kde@lemmy.kde.social

If you try to enter that link for you who are a lemmy.kde.social user you will probably get a meaningless error, but for any lemmy.dbzer0 user it will open the c/kde community from their own instance, allowing them to join c/kde even from their instance.

The syntax is like this:

https:{your instance url} c/ {name of the community} @ {url of the instance you wanna join}

Now, to make things easier for you, just click here to enter (from a web browser) and subscribe to the ubuntu community at lemmy.ml and here to see the kubuntu community at lemmy@ml, let me know if it doesn't work, at first you may not see posts because your instance needs to load.