I did the following on Jerboa

• save this post (we’ll be revisiting this a few times)
• go to the search function that is indicated by the bulleted list icon
• enter the community name (without the @… part)
• there’ll be a few results
• tap on them, see the posts if you want, and tap the subscribe button and see that it shows joined
• go to saved posts, repeat until subscribed to every community you want

Having said that, there’s probably a simpler way to do this. Still early in the learning process.