I was having a ton of difficulty with this initially following the documented steps and was getting a “404 community not found” issue for something I knew was on lemmy.world.

Convoluted so if anyone knows a better way please share (using mobile):

1. Tap the hamburger from your home server
2. Tap Communities
3. Enter minimally three characters and hit search (this should take you to a more robust search page)
4. Change the Communities drop down to All
5. Paste the url for the desired community and tap Search
6. Tap the community
7. Tap the sidebar and Subscribe

Don’t know why it doesn’t work the normal way or when the URL option is used. Only All worked for me.